ShutdownDelay should apply on shutdowns and "healthy" restarts

### Background

The documentation for [`shutdown_delay`](https://www.nomadproject.io/docs/job-specification/task.html#shutdown_delay) makes it unclear under which scenarios the delay is applied. The behavior changed in Nomad 0.9.0:

* 0.8.7 - Delay applies only when stopping a task.
* 0.9.0 - Delay applies when stopping a task *and* when restarting a task.

PR #5957 would have tweaked the behavior further:

* Delay applies when stopping a task and when restarting a "healthy" task. Restarts due to failures - such as `check_restart` induced restarts - are *not* delayed.

Unfortunately PR #5975 was accepted over #5957 because it was a more conservative approach to fixing a critical bug (#5395), and the accepted PR lacks the shutdown delay improvement.

### Enhancement

* [ ] Shutdown delay should not apply to unhealthy restarts like `check_restart`.
* [ ] Shutdown delay's documentation should be updated to reflect its restart impact.

Start by reading PR #5957, accepted PR #5975, and bug #5395 to understand the existing shutdown and restart behavior, then review the shutdown_delay documentation linked in the issue. Done means unhealthy restarts such as check_restart are not delayed and the documentation accurately describes the remaining restart impact.

Written by the indexing model from the issue text.