fix(option): update WithEndpoint docs (#3032)

fixes: googleapis/google-cloud-go#11625

Author: quartzmo

option/option.go

@@ -95,7 +95,14 @@ func (w withCredentialsJSON) Apply(o *internal.DialSettings) {
 }
 
 // WithEndpoint returns a ClientOption that overrides the default endpoint
-// to be used for a service.
+// to be used for a service. Please note that by default Google APIs only
+// accept HTTPS traffic.
+//
+// For a gRPC client, the port number is typically included in the endpoint.
+// Example: "us-central1-speech.googleapis.com:443".
+//
+// For a REST client, the port number is typically not included. Example:
+// "https://speech.googleapis.com".
 func WithEndpoint(url string) ClientOption {
 	return withEndpoint(url)
 }