docs: refresh Mergify backport guidance — single-line syntax + maintainer-only

- Update example to the consolidated `@Mergifyio backport circinus sagitta`
  form (multiple branches in one command, space-separated). The
  one-branch-per-line form still works but is no longer required.
- Document that only Maintainers team members can invoke @Mergifyio
  commands — Mergify silently drops commands from anyone outside the
  team, with no error reply. Useful failure mode to know about: if a
  backport does not trigger, check team membership first.

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