Document the allowed concurrency on Upgrader and Dialer (#636)

* Document allowed concurrency on Dialer.
* Document allowed concurrency on Upgrader.

client.go

@@ -48,6 +48,8 @@ func NewClient(netConn net.Conn, u *url.URL, requestHeader http.Header, readBufS
 }
 
 // A Dialer contains options for connecting to WebSocket server.
+//
+// It is safe to call Dialer's methods concurrently.
 type Dialer struct {
 	// NetDial specifies the dial function for creating TCP connections. If
 	// NetDial is nil, net.Dial is used.

server.go

@@ -23,6 +23,8 @@ func (e HandshakeError) Error() string { return e.message }
 
 // Upgrader specifies parameters for upgrading an HTTP connection to a
 // WebSocket connection.
+//
+// It is safe to call Upgrader's methods concurrently.
 type Upgrader struct {
 	// HandshakeTimeout specifies the duration for the handshake to complete.
 	HandshakeTimeout time.Duration