Rollup merge of #157242 - apiraino:tune-backport-zulip-messages, r=Urgau

Tune backport Zulip messages

Asked in triagebot#2407

In the zulip message that opens a backport poll, adds a suggestion about the correct triagebot syntax to approve/decline a backport to avoid ambiguity and typos

Thanks for a review

r? @Urgau