docs(option): clarify behavior of WithScopes

Clarify that when both WithScopes and WithTokenSource are used, the
scope settings will be taken from the token source and the WithScopes
option will be ignored.

Fixes googleapis#1644

option/option.go

@@ -82,6 +82,9 @@ func (w withEndpoint) Apply(o *internal.DialSettings) {
 
 // WithScopes returns a ClientOption that overrides the default OAuth2 scopes
 // to be used for a service.
+//
+// If both WithScopes and WithTokenSource are used, scope settings from the
+// token source will be used instead.
 func WithScopes(scope ...string) ClientOption {
 	return withScopes(scope)
 }