Add note about built-in extensions (microsoft#164601)

Fixes microsoft#164565

src/vscode-dts/vscode.d.ts

@@ -15638,9 +15638,13 @@ declare module 'vscode' {
 
 	/**
 	 * Namespace for localization-related functionality in the extension API. To use this properly,
-	 * you must have `l10n` defined in your `package.json` and have bundle.l10n.<language>.json files.
+	 * you must have `l10n` defined in your extension manifest and have bundle.l10n.<language>.json files.
 	 * For more information on how to generate bundle.l10n.<language>.json files, check out the
 	 * [vscode-l10n repo](https://github.com/microsoft/vscode-l10n).
+	 *
+	 * Note: Built-in extensions (for example, Git, TypeScript Language Features, GitHub Authentication)
+	 * are excluded from the `l10n` property requirement. In other words, they do not need to specify
+	 * a `l10n` in the extension manifest because their translated strings come from Language Packs.
 	 */
 	export namespace l10n {
 		/**