Clarify that the purpose of computing all the miscellaneous collection types is…

Clarify that the purpose of computing all the miscellaneous collection types is to register them with the appropriate ABCs.

Lib/collections/abc.py

@@ -18,9 +18,13 @@ __all__ = ["Hashable", "Iterable", "Iterator",
            "ByteString",
            ]
 
-
-### collection related types which are not exposed through builtin ###
-## iterators ##
+# Private list of types that we want to register with the various ABCs
+# so that they will pass tests like:
+#       it = iter(somebytearray)
+#       assert isinstance(it, Iterable)
+# Note:  in other implementations, these types many not be distinct
+# and they make have their own implementation specific types that
+# are not included on this list.
 bytes_iterator = type(iter(b''))
 bytearray_iterator = type(iter(bytearray()))
 #callable_iterator = ???