websocket: re-add documentation for DialConfig

The comment of the DialConfig function was dropped during CL 463097.
There doesn't seem to be a good reason to do that, so bring it back.

For golang/go#57953.

Change-Id: I3e458b7d18cdab95763f003da5a644c8287b54ad
Reviewed-on: https://go-review.googlesource.com/c/net/+/568198
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

websocket/client.go

@@ -79,6 +79,7 @@ func parseAuthority(location *url.URL) string {
 	return location.Host
 }
 
+// DialConfig opens a new client connection to a WebSocket with a config.
 func DialConfig(config *Config) (ws *Conn, err error) {
 	return config.DialContext(context.Background())
 }