add comment about why one UnknownIndex needs to stay (!)

Author: MarcoGorelli

pandas-stubs/core/frame.pyi

@@ -1686,6 +1686,8 @@ class DataFrame(NDFrame, OpsMixin, _GetItemHack):
     @property
     def iloc(self) -> _iLocIndexerFrame[Self]: ...
     @property
+    # mypy complains if we use Index[Any] instead of UnknownIndex here, even though
+    # the latter is aliased to the former ¯\_(ツ)_/¯.
     def index(self) -> UnknownIndex: ...
     @index.setter
     def index(self, idx: Index) -> None: ...