Document what happens when the real id is changed.

I may have obtained this from NetBSD, but I don't recall.  I do know that
I checked FreeBSD's implementation to make sure that it matched what I wrote.

lib/libc/sys/setregid.2

@@ -47,6 +47,9 @@
 .Sh DESCRIPTION
 The real and effective group ID's of the current process
 are set to the arguments.
+If the real group ID is changed, the saved group ID is changed to the
+new value of the effective group ID.
+.Pp
 Unprivileged users may change the real group
 ID to the effective group ID and vice-versa; only the super-user may
 make other changes.

lib/libc/sys/setreuid.2

@@ -47,6 +47,9 @@
 .Sh DESCRIPTION
 The real and effective user IDs of the
 current process are set according to the arguments.
+If the real user ID is changed, the saved user ID is changed to the
+new value of the effective user ID.
+.Pp
 If
 .Fa ruid
 or