docs: add `win.isFocusable()` return type (#37245)

docs: fix `win.isFocusable()` return type

docs/api/browser-window.md

@@ -1776,7 +1776,7 @@ On macOS it does not remove the focus from the window.
 
 #### `win.isFocusable()` _macOS_ _Windows_
 
-Returns whether the window can be focused.
+Returns `boolean` - Whether the window can be focused.
 
 #### `win.setParentWindow(parent)`