bpo-22005: Document the reality of pickle compatibility. (GH-11054)

Doc/library/pickle.rst

@@ -71,7 +71,9 @@ The :mod:`pickle` module differs from :mod:`marshal` in several significant ways
   :file:`.pyc` files, the Python implementers reserve the right to change the
   serialization format in non-backwards compatible ways should the need arise.
   The :mod:`pickle` serialization format is guaranteed to be backwards compatible
-  across Python releases.
+  across Python releases provided a compatible pickle protocol is chosen and
+  pickling and unpickling code deals with Python 2 to Python 3 type differences
+  if your data is crossing that unique breaking change language boundary.
 
 Comparison with ``json``
 ^^^^^^^^^^^^^^^^^^^^^^^^