add comment to copyWithZone_ about the retain call

cocoa/src/toga_cocoa/widgets/internal/data.py

@@ -5,6 +5,9 @@ class TogaData(NSObject):
     @objc_method
     def copyWithZone_(self):
         # TogaData is used as an immutable reference to a row
-        # so the same object can be returned as a copy.
+        # so the same object can be returned as a copy. We need
+        # to manually `retain` the object before returning because
+        # the "copy" methods are assumed to return an object that
+        # is owned by the caller.
         self.retain()
         return self