docs/doc-guide: Put meta title for kernel-doc HTML page

kernel-doc.rst has two 1st level section titles of "Writing
kernel-doc comments" and "Including kernel-doc comments".

Therefore, rather than using the first one, put a meta title
of "Kernel-doc comments" for the title of the resulting HTML
page by using the "title" directive.
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/15636c13-7fa2-f973-6d3d-361222b839ef@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/doc-guide/kernel-doc.rst

+.. title:: Kernel-doc comments
+
 ===========================
 Writing kernel-doc comments
 ===========================