Improves comments

session.go

@@ -179,6 +179,7 @@ func (s *Session) rfc3921Session(o Config) {
 	}
 
 	var iq stanza.IQ
+	// We only negotiate session binding if it is mandatory, we skip it when optional.
 	if !s.Features.Session.IsOptional() {
 		fmt.Fprintf(s.streamLogger, "<iq type='set' id='%s'><session xmlns='%s'/></iq>", s.PacketId(), stanza.NSSession)
 		if s.err = s.decoder.Decode(&iq); s.err != nil {

stanza/sasl_auth.go

@@ -99,6 +99,7 @@ func (s *StreamSession) IsOptional() bool {
 	if s.XMLName.Local == "session" {
 		return s.Optional
 	}
+	// If session element is missing, then we should not use session
 	return true
 }