msg_callback documentation

doc/apps/s_server.pod

@@ -21,6 +21,7 @@ B<openssl> B<s_server>
 [B<-nbio_test>]
 [B<-crlf>]
 [B<-debug>]
+[B<-msg>]
 [B<-state>]
 [B<-CApath directory>]
 [B<-CAfile filename>]
@@ -136,6 +137,10 @@ 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