Add doc to resolveTreeItem

Part of #100741

src/vs/vscode.proposed.d.ts

@@ -1001,6 +1001,16 @@ declare module 'vscode' {
 
 	// https://github.com/microsoft/vscode/issues/100741
 	export interface TreeDataProvider<T> {
+		/**
+		 * Called only on hover to resolve the TreeItem2#tooltip property if it is undefined.
+		 * Only properties that were undefined can be resolved in `resolveTreeItem`.
+		 * Will only ever be called once per TreeItem.
+		 * Functionality may be expanded later to include being called to resolve other missing
+		 * properties on selection and/or on open.
+		 *
+		 * @param element
+		 * @param item Undefined properties of `item` should be set then `item` should be returned.
+		 */
 		resolveTreeItem?(element: T, item: TreeItem2): TreeItem2 | Thenable<TreeItem2>;
 	}