Add a comment about collections.abc vs typing

See https://docs.python.org/3/library/typing.html#typing.AbstractSet

PiperOrigin-RevId: 683161346

pytype/overlays/collections_overlay.py

@@ -33,4 +33,7 @@ class ABCOverlay(typing_overlay.Redirect):
   """A custom overlay for the 'collections.abc' module."""
 
   def __init__(self, ctx):
-    super().__init__("collections.abc", {"Set": "typing.AbstractSet"}, ctx)
+    # collections.abc.Set equates to typing.AbstractSet rather than typing.Set.
+    # This is the only such mismatch.
+    aliases = {"Set": "typing.AbstractSet"}
+    super().__init__("collections.abc", aliases, ctx)