`graphile-migrate watch --once` is hard to discover

When you run `graphile-migrate --help` there's no hint that `watch --once` exists, and the description for `watch` doesn't seem like what you need.

Suggestions:

- [ ] Update the `watch` description so it also covers the `--once` use case
- [ ] Consider adding a command alias (identical to calling `graphile-migrate watch --once`) to raise this to a top-level concern; e.g. `graphile-migrate run-current` or `graphile-migrate watch-once` or similar (suggestions welcome, I'm currently leaning towards `watch-once`)

> Really it's "run all migrations plus the current migration and then exit"
> Or "watch mode but without the watching..." :wink:
> -- @Benjie