One question is how spin this. Listing a long series of incompatibilities presents the image of a language that has many gratuitous incompatible changes -- which is not true and the opposite of the message we want to convey.
This way madness lies. Document nothing and there will certainly be complaints. Why worry about losers who are in essence complaining that the documentation is too complete?
All I'm asking is a leading paragraph something like this:
"""While the Python developers do their best to make every subsequent release backwards compatible with previous releases, in a system of this complexity, sometimes incompatibilities in odd corners of the language are unavoidable. While we expect that these will affect only a small minority of Python users, we document them here for completeness."""
--Guido van Rossum (home page: http://www.python.org/~guido/)