macOS: Clarify useless-looking code in VLCDocumentController

Clarify why the VLCDocumentController is allocated and initialized in
the load method, so other developers are not confused by or accidentally
delete it, as it looks useless.
Additionally do not assign to an unused variable but just cast the
result to void, to make clear it is unused on purpose.

modules/gui/macosx/VLCDocumentController.m

@@ -27,7 +27,11 @@
 
 + (void)load
 {
-    id controller = [[VLCDocumentController alloc] init];
+    /* For the custom document controller to be used by the app,
+     * it needs to be created before the default NSDocumentController
+     * is created.
+     */
+    (void)[[VLCDocumentController alloc] init];
 }
 
 - (IBAction)clearRecentDocuments:(id)sender