Document that webviews do not let you post blobs or other dom objects

Fixes #79257

src/vs/vscode.d.ts

@@ -5912,6 +5912,10 @@ declare module 'vscode' {
 
 		/**
 		 * Fired when the webview content posts a message.
+		 *
+		 * Webview content can post strings or json serilizable objects back to a VS Code extension. They cannot
+		 * post `Blob`, `File`, `ImageData` and other DOM specific objects since the extension that receives the
+		 * message does not run in a browser environment.
 		 */
 		readonly onDidReceiveMessage: Event<any>;
 
@@ -5921,7 +5925,7 @@ declare module 'vscode' {
 		 * Messages are only delivered if the webview is live (either visible or in the
 		 * background with `retainContextWhenHidden`).
 		 *
-		 * @param message Body of the message.
+		 * @param message Body of the message. This must be a string or other json serilizable object.
 		 */
 		postMessage(message: any): Thenable<boolean>;