docs: 5.Posting.rst: describe Fixes: and Link: tags

Explain Fixes: and Link: tags in Documentation/process/5.Posting.rst,
which are missing in this file for unknown reasons and only described in
Documentation/process/submitting-patches.rst.
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
CC: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://lore.kernel.org/r/c4a5f5e25fa84b26fd383bba6eafde4ab57c9de7.1641314856.git.linux@leemhuis.infoSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/5.Posting.rst

@@ -197,14 +197,29 @@ the build process, for example, or editor backup files) in the patch.  The
 file "dontdiff" in the Documentation directory can help in this regard;
 pass it to diff with the "-X" option.
 
-The tags mentioned above are used to describe how various developers have
-been associated with the development of this patch.  They are described in
-detail in
-the :ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
-document; what follows here is a brief summary.  Each of these lines has
-the format:
+The tags already briefly mentioned above are used to provide insights how
+the patch came into being. They are described in detail in the
+:ref:`Documentation/process/submitting-patches.rst <submittingpatches>`
+document; what follows here is a brief summary.
 
-::
+One tag is used to refer to earlier commits which introduced problems fixed by
+the patch::
+
+	Fixes: 1f2e3d4c5b6a ("The first line of the commit specified by the first 12 characters of its SHA-1 ID")
+
+Another tag is used for linking web pages with additional backgrounds or
+details, for example a report about a bug fixed by the patch or a document
+with a specification implemented by the patch::
+
+	Link: https://example.com/somewhere.html  optional-other-stuff
+
+Many maintainers when applying a patch also add this tag to link to the
+latest public review posting of the patch; often this is automatically done
+by tools like b4 or a git hook like the one described in
+'Documentation/maintainer/configure-git.rst'.
+
+A third kind of tag is used to document who was involved in the development of
+the patch. Each of these uses this format::
 
 	tag: Full Name <email address>  optional-other-stuff