doc: security: Add kern-doc for lsm_hooks.h

There is a lot of kern-doc for the LSM internals, but it wasn't visible
in the HTML output. This exposes some formatting flaws in lsm_hooks.h
that will be fixed in a later series of patches.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>