LSM: fix documentation for the syslog hook

The syslog hook was changed in the commit
"capabilities/syslog: open code cap_syslog logic to
fix build failure" (12b3052c

). The argument @from_file
was removed from the hook. This patch updates the
documentation for the syslog hook accordingly.

Signed-off-by: Denis Efremov <efremov@ispras.ru>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: James Morris <james.morris@microsoft.com>