msg_callback documentation

doc/apps/s_client.pod

@@ -18,6 +18,7 @@ B<openssl> B<s_client>
 [B<-pause>]
 [B<-showcerts>]
 [B<-debug>]
+[B<-msg>]
 [B<-nbio_test>]
 [B<-state>]
 [B<-nbio>]
@@ -112,6 +113,10 @@ prints out the SSL session states.
 
 print extensive debugging information including a hex dump of all traffic.
 
+=item B<-msg>
+
+show all protocol messages with hex dump.
+
 =item B<-nbio_test>
 
 tests non-blocking I/O