doc: note on release guide to update main branch

Added an extra command to make sure to remind me (and other releasers)
to update the `main` branch _before_ cherry-picking the release commit
to the branch.

Adding this command will often avoid unnecessary extra-work in case
there are mismatches between the local state of the main branch and
remote.

PR-URL: #44384
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

doc/contributing/releases.md

@@ -683,6 +683,7 @@ repository.
 
 ```console
 $ git checkout main
+$ git pull upstream main
 $ git cherry-pick v1.x^
 ```