docs: Get rid of some kernel-documentation warnings

Sphinx wants to interpret all literal blocks as being in the chosen
language and complains when an attempt to parse a block fails.
kernel-documentation.rst has a few blocks that are not in C; make that
explicit to shut down the associated warnings.
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kernel-documentation.rst

@@ -366,6 +366,8 @@ Domain`_ references.
 Cross-referencing from reStructuredText
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+.. highlight:: none
+
 To cross-reference the functions and types defined in the kernel-doc comments
 from reStructuredText documents, please use the `Sphinx C Domain`_
 references. For example::
@@ -388,6 +390,8 @@ For further details, please refer to the `Sphinx C Domain`_ documentation.
 Function documentation
 ----------------------
 
+.. highlight:: c
+
 The general format of a function and function-like macro kernel-doc comment is::
 
   /**
@@ -568,6 +572,8 @@ DocBook XML [DEPRECATED]
 Converting DocBook to Sphinx
 ----------------------------
 
+.. highlight:: none
+
 Over time, we expect all of the documents under ``Documentation/DocBook`` to be
 converted to Sphinx and reStructuredText. For most DocBook XML documents, a good
 enough solution is to use the simple ``Documentation/sphinx/tmplcvt`` script,