docs: client-credentials is not support by all backends

Author: edc-w

lib/oauthutil/oauthutil.go

@@ -155,7 +155,7 @@ var SharedOptions = []fs.Option{{
 }, {
 	Name:     config.ConfigClientCredentials,
 	Default:  false,
-	Help:     "Use client credentials OAuth flow.\n\nThis will use the OAUTH2 client Credentials Flow as described in RFC 6749.",
+	Help:     "Use client credentials OAuth flow.\n\nThis will use the OAUTH2 client Credentials Flow as described in RFC 6749.\n\nNote that this option is NOT supported by all backends.",
 	Advanced: true,
 }}