doc-guide: kernel-doc: add comment about formatting verification

Currently there is no automated checking for kernel-doc comments except
running 'kernel-doc -v -none <filename>'. Mention the possibility to run
kernel-doc to verify formatting of the comments in the kernel-doc guide.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>