tweak release checklist

Signed-off-by: Victor Vieux <vieux@docker.com>
Until now, we've used `docs` branch as a rolling docs upgrade to `release` - so whenever a major release happens, it seems right to reset to `release` - and I think we would need to --force for that to work.

If we were decide to change the docs release branches to *not* using `docs` but rather go straight to `docs-$MAJOR.$MINOR`, then we avoid this kind of thing.