Usually my technique is to apply the 3.6 patch to 3.5, fix up any conflicts, and leave the 3.6-only bits out (which get rejected by the patch process anyway). But dedicated patch(es) may be useful. Especially for 2.7, where there are probably independent changes to be made (e.g. modules that were removed in Python 3).

I think the policy on documentation in each branch should be in the devguide. My understanding: In general, bug fix branches (3.5, 2.7) have the documentation maintained, but generally not the older security-only branches.

My view is if a feature is added to (say) 3.6, then it gets a new-in-3.6 notice in the 3.6 documentation, but nothing gets added to 3.5. The Python 3 documentation rarely mentions features of Python 2, so every feature is treated as being new in 3.0 by default. For Python 2, it won’t document new features of Python 3, but is updated with changes relevant to porting to 3. So in theory the latest Python 3.6+ documentation should also be usable with 3.5, but not with Python 2. That’s about all I know :)