MAINTAINERS: mention documentation maintainer entry profile

Since commit 53b7f3a ("Add a maintainer entry profile for
documentation"), the documentation "subsystem" has a maintainer entry
profile, and it deserves to be mentioned in MAINTAINERS with a suitable
P: entry.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200815102658.12236-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: bulwahn

MAINTAINERS

@@ -5239,6 +5239,7 @@ DOCUMENTATION
 M:	Jonathan Corbet <corbet@lwn.net>
 L:	linux-doc@vger.kernel.org
 S:	Maintained
+P:	Documentation/doc-guide/maintainer-profile.rst
 T:	git git://git.lwn.net/linux.git docs-next
 F:	Documentation/
 F:	scripts/documentation-file-ref-check