Smack: Improve Documentation

Add some words about the mount option "smackfstransmute=label".
Signed-off-by: José Bollo <jobol@nonadev.net>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

Documentation/admin-guide/LSM/Smack.rst

@@ -818,6 +818,10 @@ Smack supports some mount options:
 	specifies a label to which all labels set on the
 	filesystem must have read access. Not yet enforced.
 
+  smackfstransmute=label:
+	behaves exactly like smackfsroot except that it also
+	sets the transmute flag on the root of the mount
+
 These mount options apply to all file system types.
 
 Smack auditing