maint: Add further details about merging topics to release branch

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.

Help/dev/maint.rst

@@ -51,15 +51,18 @@ using a local branch named ``release-$ver``, where ``$ver`` is the version
 number of the current release in the form ``$major.$minor``.  It is always
 merged into ``master`` before publishing.
 
-To merge some ``$topic`` branch into ``release``, first create the local
-branch:
+Before merging a ``$topic`` branch into ``release``, verify that the
+``$topic`` branch has already been merged to ``master`` via the usual
+``Do: merge`` process.  Then, to merge the ``$topic`` branch into
+``release``, start by creating the local branch:
 
 .. code-block:: shell
 
   git fetch origin
   git checkout -b release-$ver origin/release
 
-Merge the ``$topic`` branch into the local ``release-$ver`` branch:
+Merge the ``$topic`` branch into the local ``release-$ver`` branch, making
+sure to include a ``Merge-request: !xxxx`` footer in the commit message:
 
 .. code-block:: shell