Add documentation for Context section

This section is mentioned in scripts/kernel-doc, so we should mention it
in doc-guide/kernel-doc.rst.  There are close to 500 comments using the
Context section already, and almost 300 using a Locking section which
fulfills much the same purpose (and should probably be converted for
consistency).

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>