Exactly. The changes _are_ documented in detail, indeed, but in special documents which only serious users read when they about to migrate from one version to another. I'm thinking about users who use the language occasionally or even regularly, but not fanatic about following everything about versions -- they mainly rely on the Python Library Reference, or even the Python Language Reference.
Hm. People who don't read the detailed documents shouldn't expect to rely on details.
These references describe some Python, but not necessarily the Python which happens to run on a given machine, and I guess (without having really experienced this myself) it might be frustrating to read and study, for discovering soon after that the feature is unavailable in this version. Or even, for someone, to have handy information about how to write for the common denominator, without having to compare many printings of the references at various Python levels. I guess that notes or footnotes, about Python levels in which described features have been implemented, might help users having to cope with release lags between Linux releases. Such lags are unavoidable whenever Python evolves.
Have you read the library manual recently? We are very careful in adding notes about which version added a particular feature or even detail. So I think there is no reason to complain about this preemptively unless your own experience indicates there's a lack of documentation.
--Guido van Rossum (home page: http://www.python.org/~guido/)