Improve control message handler doc

Fixes #338

conn.go

@@ -1051,8 +1051,9 @@ func (c *Conn) CloseHandler() func(code int, text string) error {
 // if the close message is empty. The default close handler sends a close
 // message back to the peer.
 //
-// The application must read the connection to process close messages as
-// described in the section on Control Messages above.
+// The handler function is called from the NextReader, ReadMessage and message
+// reader Read methods. The application must read the connection to process
+// close messages as described in the section on Control Messages above.
 //
 // The connection read methods return a CloseError when a close message is
 // received. Most applications should handle close messages as part of their
@@ -1079,8 +1080,9 @@ func (c *Conn) PingHandler() func(appData string) error {
 // The appData argument to h is the PING message application data. The default
 // ping handler sends a pong to the peer.
 //
-// The application must read the connection to process ping messages as
-// described in the section on Control Messages above.
+// The handler function is called from the NextReader, ReadMessage and message
+// reader Read methods. The application must read the connection to process
+// ping messages as described in the section on Control Messages above.
 func (c *Conn) SetPingHandler(h func(appData string) error) {
 	if h == nil {
 		h = func(message string) error {
@@ -1105,8 +1107,9 @@ func (c *Conn) PongHandler() func(appData string) error {
 // The appData argument to h is the PONG message application data. The default
 // pong handler does nothing.
 //
-// The application must read the connection to process ping messages as
-// described in the section on Control Messages above.
+// The handler function is called from the NextReader, ReadMessage and message
+// reader Read methods. The application must read the connection to process
+// pong messages as described in the section on Control Messages above.
 func (c *Conn) SetPongHandler(h func(appData string) error) {
 	if h == nil {
 		h = func(string) error { return nil }