Under parallel development, when something is worth being commented
in upgrade.txt notes, it will be applied to all the development branches
using the "earliest" one to be released. So, if something changes
in 3.10 (and also is applied to master), the upgrade.txt notes will
show 3.10 in both branches.

Of course, if something only goes to one dev branch (say master), then the
master version will be used in the notes (4.0 in this case).