Add description proposed dts file for #33358

Author: octref

src/vs/vscode.proposed.d.ts

@@ -3,7 +3,16 @@
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
 
-// This is the place for API experiments and proposals.
+/**
+ * This is the place for API experiments and proposals.
+ * These API are NOT stable and subject to change. They are only available in the Insiders
+ * distribution and CANNOT be used in published extensions.
+ *
+ * To test these API in local environment:
+ * - Use Insiders release of VS Code.
+ * - Add `"enableProposedApi": true` to your package.json.
+ * - Copy this file to your project.
+ */
 
 declare module 'vscode' {