Documentation: kernel-doc: Promote two chapter headings to page title

Promote "Writing kernel-doc comments" heading to page title, in
accordance to kernel documentation guidelines. Also promote "Including
kernel-doc comments" heading because both headings deserve their own
chapters in PDF output.

There is no differences in the resulting output except formatting
semantics.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Tony Nguyen <anthony.l.nguyen@intel.com>
Cc: Vinod Koul <vkoul@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/doc-guide/kernel-doc.rst

+===========================
 Writing kernel-doc comments
 ===========================
 
@@ -436,6 +437,7 @@ The title following ``DOC:`` acts as a heading within the source file, but also
 as an identifier for extracting the documentation comment. Thus, the title must
 be unique within the file.
 
+=============================
 Including kernel-doc comments
 =============================