doc: security: minor cleanups to build kernel-doc

These fixes were needed to parse lsm_hooks.h kernel-doc. More work is
needed, but this is the first step.

Acked-by: James Morris <james.l.morris@oracle.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>