mus: Add comment to PointerWatcher

ui/views/pointer_watcher.h

Index: ui/views/pointer_watcher.h
diff --git a/ui/views/pointer_watcher.h b/ui/views/pointer_watcher.h
index 6273e130c0725f251a1f7fb0e869d237ae405e0b..dc7fa8de9b88255d6f9dec223a8e2d9e3434d7ce 100644
--- a/ui/views/pointer_watcher.h
+++ b/ui/views/pointer_watcher.h
@@ -16,6 +16,8 @@ namespace views {
 
 // An interface for read-only observation of pointer events (in particular, the
 // events cannot be marked as handled). Only certain event types are supported.
+// NOTE: The event.target is always null, because on mus the target window may
+// be owned by another process.
 class VIEWS_EXPORT PointerWatcher {
  public:
   virtual ~PointerWatcher() {}