It wasn't clear to me after reading the "Forward Porting" section exactly what was going on.  Nick Coghlan spelled it out for me in a private email, and suggested that maybe this stuff should be in the devguide proper.  Here's some specific stuff that I didn't understand until Nick explained it to me with simple words:

* 2.x and 3.x have separate heads in the same repository
* Since they're totally divorced, the order you check in to 2.x and 3.x does not matter
* DO NOT MERGE between 2.x and 3.x
* Branches that are in security-fix-only mode (e.g. 3.1) don't get bugfixes or documentation fixes (surely mentioned elsewhere, but I personally would have been helped with a reminder)

I suggest it'd be clearer to start with discussing "2.x and 3.x are separate heads", and *then* move on to "But when merging changes solely inside a major version" and talk about forward-porting.

Would you be interested in a patch?