executors: fix and move no-forward-progress alert

Created by: Strum355

Somehow the alert query in https://github.com/sourcegraph/sourcegraph/pull/40570 got merged without the query being right, maybe a consequence of context switching. So this PR just fixes it up and also moves it to a more relevant panel (active handlers instead of jobs completed).

Adds some additional generated doc configuration for alert description (mostly useful in conjunction with custom alert query facility) for DevX to review

Test plan

Correctness update for something that was already tested before