lsm: move the AF_UNIX hook comments to security/security.c

lsm: move the AF_UNIX hook comments to security/security.c

This patch relocates the LSM hook function comments to the function
definitions, in keeping with the current kernel conventions.  This
should make the hook descriptions more easily discoverable and easier
to maintain.

While formatting changes have been done to better fit the kernel-doc
style, content changes have been kept to a minimum and limited to
text which was obviously incorrect and/or outdated.  It is expected
the future patches will improve the quality of the function header
comments.

Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>