Improve server subprotocol documentation

Partial fix for 404.

server.go

@@ -34,9 +34,11 @@ type Upgrader struct {
	ReadBufferSize, WriteBufferSize int

	// Subprotocols specifies the server's supported protocols in order of
	// preference. If this field is set, then the Upgrade method negotiates a
	// preference. If this field is not nil, then the Upgrade method negotiates a
	// subprotocol by selecting the first match in this list with a protocol
	// requested by the client.
	// requested by the client. If there's no match, then no protocol is
	// negotiated (the Sec-Websocket-Protocol header is not included in the
	// handshake response).
	Subprotocols []string

	// Error specifies the function for generating HTTP error responses. If Error