kernel-doc: rewrite usage description, remove duplicated comments

Instead of having the kernel-doc usage in both comments and in output to
the user, merge them all to one here document. While at it, imrove the
text and make it pretty. Give shoemaker's children some shoes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>