docs: kerneldoc-preamble.sty: Expand comments in LaTeX code

Expand comments in LaTeX code and mention some of important points
told in changelogs of conf.py changes.
Hopefully they can help future contributors in this area.

No code change involved.
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/bce9261b-1950-3146-07b2-07bd2ec79158@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sphinx/kerneldoc-preamble.sty

@@ -15,8 +15,20 @@
 %
 % Copyright (C) 2022  Akira Yokosawa
 
-% Custom width parameters for TOC --- Redefine low-level commands
-% defined in report.cls
+% Custom width parameters for TOC
+%  - Redefine low-level commands defined in report.cls.
+%  - Indent of 2 chars is preserved for ease of comparison.
+% Summary of changes from default params:
+%   Width of page number (\@pnumwidth): 1.55em -> 2.7em
+%   Width of chapter number:            1.5em  -> 1.8em
+%   Indent of section number:           1.5em  -> 1.8em
+%   Width of section number:            2.6em  -> 3.2em
+%   Indent of sebsection number:        4.1em  -> 5em
+%   Width of subsection number:         3.5em  -> 4.3em
+%
+% These params can have 4 digit page counts, 2 digit chapter counts,
+% section counts of 4 digits + 1 period (e.g., 18.10), and subsection counts
+% of 5 digits + 2 periods (e.g., 18.7.13).
 \makeatletter
 %% Redefine \@pnumwidth (page number width)
 \renewcommand*\@pnumwidth{2.7em}
@@ -46,7 +58,10 @@
 \providecommand{\sphinxtableofcontentshook}{}
 %% Undefine it for compatibility with Sphinx 1.7.9
 \renewcommand{\sphinxtableofcontentshook}{} % Empty the hook
-% Prevent column squeezing of tabulary.
+
+% Prevent column squeezing of tabulary.  \tymin is set by Sphinx as:
+%   \setlength{\tymin}{3\fontcharwd\font`0 }
+% , which is too short.
 \setlength{\tymin}{20em}
 
 % Adjust \headheight for fancyhdr
@@ -56,8 +71,12 @@
 % Translations have Asian (CJK) characters which are only displayed if
 % xeCJK is used
 \IfFontExistsTF{Noto Sans CJK SC}{
-    % This is needed for translations
+    % Load xeCJK when CJK font is available
     \usepackage{xeCJK}
+    % Noto CJK fonts don't provide slant shape. [AutoFakeSlant] permits
+    % its emulation.
+    % Select KR variant at the beginning of each document so that quotation
+    % and apostorph symbols of half-width is used in TOC of Latin documents.
     \IfFontExistsTF{Noto Serif CJK KR}{
 	\setCJKmainfont{Noto Serif CJK KR}[AutoFakeSlant]
     }{
@@ -65,9 +84,11 @@
     }
     \setCJKsansfont{Noto Sans CJK KR}[AutoFakeSlant]
     \setCJKmonofont{Noto Sans Mono CJK KR}[AutoFakeSlant]
+    % Teach xeCJK of half-width symbols
     \xeCJKDeclareCharClass{HalfLeft}{`“,`‘}
     \xeCJKDeclareCharClass{HalfRight}{`”,`’}
     % CJK Language-specific font choices
+    %% for Simplified Chinese
     \IfFontExistsTF{Noto Serif CJK SC}{
 	\newCJKfontfamily[SCmain]\scmain{Noto Serif CJK SC}[AutoFakeSlant]
 	\newCJKfontfamily[SCserif]\scserif{Noto Serif CJK SC}[AutoFakeSlant]
@@ -77,6 +98,7 @@
     }
     \newCJKfontfamily[SCsans]\scsans{Noto Sans CJK SC}[AutoFakeSlant]
     \newCJKfontfamily[SCmono]\scmono{Noto Sans Mono CJK SC}[AutoFakeSlant]
+    %% for Traditional Chinese
     \IfFontExistsTF{Noto Serif CJK TC}{
 	\newCJKfontfamily[TCmain]\tcmain{Noto Serif CJK TC}[AutoFakeSlant]
 	\newCJKfontfamily[TCserif]\tcserif{Noto Serif CJK TC}[AutoFakeSlant]
@@ -86,6 +108,7 @@
     }
     \newCJKfontfamily[TCsans]\tcsans{Noto Sans CJK TC}[AutoFakeSlant]
     \newCJKfontfamily[TCmono]\tcmono{Noto Sans Mono CJK TC}[AutoFakeSlant]
+    %% for Korean
     \IfFontExistsTF{Noto Serif CJK KR}{
 	\newCJKfontfamily[KRmain]\krmain{Noto Serif CJK KR}[AutoFakeSlant]
 	\newCJKfontfamily[KRserif]\krserif{Noto Serif CJK KR}[AutoFakeSlant]
@@ -95,6 +118,7 @@
     }
     \newCJKfontfamily[KRsans]\krsans{Noto Sans CJK KR}[AutoFakeSlant]
     \newCJKfontfamily[KRmono]\krmono{Noto Sans Mono CJK KR}[AutoFakeSlant]
+    %% for Japanese
     \IfFontExistsTF{Noto Serif CJK JP}{
 	\newCJKfontfamily[JPmain]\jpmain{Noto Serif CJK JP}[AutoFakeSlant]
 	\newCJKfontfamily[JPserif]\jpserif{Noto Serif CJK JP}[AutoFakeSlant]
@@ -108,34 +132,39 @@
     \providecommand{\onehalfspacing}{}
     \providecommand{\singlespacing}{}
     % Define custom macros to on/off CJK
+    %% One and half spacing for CJK contents
     \newcommand{\kerneldocCJKon}{\makexeCJKactive\onehalfspacing}
     \newcommand{\kerneldocCJKoff}{\makexeCJKinactive\singlespacing}
+    % Define custom macros for switching CJK font setting
+    %% for Simplified Chinese
     \newcommand{\kerneldocBeginSC}{%
 	\begingroup%
 	\scmain%
-	\xeCJKDeclareCharClass{FullLeft}{`“,`‘}%
-	\xeCJKDeclareCharClass{FullRight}{`”,`’}%
+	\xeCJKDeclareCharClass{FullLeft}{`“,`‘}% Full-width in SC
+	\xeCJKDeclareCharClass{FullRight}{`”,`’}% Full-width in SC
 	\renewcommand{\CJKrmdefault}{SCserif}%
 	\renewcommand{\CJKsfdefault}{SCsans}%
 	\renewcommand{\CJKttdefault}{SCmono}%
-	\xeCJKsetup{CJKspace = false}%
+	\xeCJKsetup{CJKspace = false}% gobble white spaces by ' '
 	% For CJK ascii-art alignment
 	\setmonofont{Noto Sans Mono CJK SC}[AutoFakeSlant]%
     }
     \newcommand{\kerneldocEndSC}{\endgroup}
+    %% for Traditional Chinese
     \newcommand{\kerneldocBeginTC}{%
 	\begingroup%
 	\tcmain%
-	\xeCJKDeclareCharClass{FullLeft}{`“,`‘}%
-	\xeCJKDeclareCharClass{FullRight}{`”,`’}%
+	\xeCJKDeclareCharClass{FullLeft}{`“,`‘}% Full-width in TC
+	\xeCJKDeclareCharClass{FullRight}{`”,`’}% Full-width in TC
 	\renewcommand{\CJKrmdefault}{TCserif}%
 	\renewcommand{\CJKsfdefault}{TCsans}%
 	\renewcommand{\CJKttdefault}{TCmono}%
-	\xeCJKsetup{CJKspace = false}%
+	\xeCJKsetup{CJKspace = false}% gobble white spaces by ' '
 	% For CJK ascii-art alignment
 	\setmonofont{Noto Sans Mono CJK TC}[AutoFakeSlant]%
     }
     \newcommand{\kerneldocEndTC}{\endgroup}
+    %% for Korean
     \newcommand{\kerneldocBeginKR}{%
 	\begingroup%
 	\krmain%
@@ -147,29 +176,35 @@
 	\setmonofont{Noto Sans Mono CJK KR}[AutoFakeSlant]%
     }
     \newcommand{\kerneldocEndKR}{\endgroup}
+    %% for Japanese
     \newcommand{\kerneldocBeginJP}{%
 	\begingroup%
 	\jpmain%
 	\renewcommand{\CJKrmdefault}{JPserif}%
 	\renewcommand{\CJKsfdefault}{JPsans}%
 	\renewcommand{\CJKttdefault}{JPmono}%
-	\xeCJKsetup{CJKspace = false}%
+	\xeCJKsetup{CJKspace = false}% gobble white space by ' '
 	% For CJK ascii-art alignment
 	\setmonofont{Noto Sans Mono CJK JP}[AutoFakeSlant]%
     }
     \newcommand{\kerneldocEndJP}{\endgroup}
+
     % Single spacing in literal blocks
     \fvset{baselinestretch=1}
     % To customize \sphinxtableofcontents
     \usepackage{etoolbox}
     % Inactivate CJK after tableofcontents
     \apptocmd{\sphinxtableofcontents}{\kerneldocCJKoff}{}{}
-    \xeCJKsetup{CJKspace = true} % For inter-phrase space of Korean TOC
+    \xeCJKsetup{CJKspace = true}% For inter-phrase space of Korean TOC
 }{ % No CJK font found
-    % Custom macros to on/off CJK (Dummy)
+    % Custom macros to on/off CJK and switch CJK fonts (Dummy)
     \newcommand{\kerneldocCJKon}{}
     \newcommand{\kerneldocCJKoff}{}
+    %% By defining \kerneldocBegin(SC|TC|KR|JP) as commands with an argument
+    %% and ignore the argument (#1) in their definitions, whole contents of
+    %% CJK chapters can be ignored.
     \newcommand{\kerneldocBeginSC}[1]{%
+	%% Put a note on missing CJK fonts in place of zh_CN translation.
 	\begin{sphinxadmonition}{note}{Note:}
 	    ``Noto Sans CJK'' fonts are not found while building this PDF\@.
 	    Translations of zh\_CN, zh\_TW, ko\_KR, and ja\_JP are skipped.