MODSIGN: Fix documentation of signed-nokey behavior when not enforcing.

jwboyer's previous commit changes the behavior of module signing when
there's a valid signature but we don't know the public key and are in
permissive mode.  This updates the documentation to match.

Signed-off-by: Peter Jones <pjones@redhat.com>
Acked-by: Josh Boyer <jwboyer@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>