Add comment about 3rd party integrations

shells/browser/shared/src/renderer.js

@@ -15,6 +15,9 @@ Object.defineProperty(
   '__REACT_DEVTOOLS_ATTACH__',
   ({
     enumerable: false,
+    // This property needs to be configurable to allow third-party integrations
+    // to attach their own renderer. Note that using third-party integrations
+    // is not officially supported. Use at your own risk.
     configurable: true,
     get() {
       return attach;