RMG - Add a note about merging the release branch back into blead

I was momentarily confused and almost did the wrong thing when I saw that
"git merge release-5.19.4" had produced a merge commit (71d5a36340),
whereas the merge of 5.19.3 a month ago did not (see f865d60069).

The cause was simply that a change (ac239e1c3e) had been pushed to blead
since I'd created the release branch, which had not happened when merging
the 5.19.3 release branch last month.