Release process: clarify what to do wrt. feature/buster.

I prefer the RM to leave it alone than mess it up, so let's give them the option
to decide it's too complicated :)

wiki/src/contribute/release_process.mdwn

@@ -297,6 +297,9 @@ Update other base branches
    [[merging base branches|APT_repository/custom#workflow-merge-main-branch]].
    (For now `feature/buster` is handled as any other topic branch
    forked off `devel`: its base branch is set to `devel`.)
+   If the merge conflicts don't look like something you feel confident
+   resolving properly, abort this merge and let the Foundations
+   Team know.
 
 4. Ensure that the release, `devel` and `feature/buster` branches
    have the expected content in `config/APT_overlays.d/`: e.g. it must