The Maintain Current Release instructions previously assumed that
the topic branch had been merged to master. Add text to make this
explicit in the instructions as an initial verification step.
The previous instructions also made no mention of the commit
message for the merge to the release branch. This needs to include
a footer that mentions the merge request number for tracking
purposes.
Extend the maintainer guide with a reminder to self-review the
new ancestry of the `release` and `master` branches to ensure
that nothing unexpected was merged.
