Improve NextWriter documentation

Author: garyburd

conn.go

@@ -484,6 +484,9 @@ func (c *Conn) prepWrite(messageType int) error {
 //
 // There can be at most one open writer on a connection. NextWriter closes the
 // previous writer if the application has not already done so.
+//
+// All message types (TextMessage, BinaryMessage, CloseMessage, PingMessage and
+// PongMessage) are supported.
 func (c *Conn) NextWriter(messageType int) (io.WriteCloser, error) {
 	if err := c.prepWrite(messageType); err != nil {
 		return nil, err