[docs] Explain cleanup of the allocation directory strategy

[gpaggi]

I found #755 to address the client allocation directory cleanup, as referenced in the change log for 0.3.0, but there's no mention in the docs about how that works.
It'd be great to have a couple of lines explaining what is the cleanup strategy.

I've run a small test and it looks like that if an allocation never enters the running state and the job gets stopped, the allocation directory doesn't get cleaned up.

[stale]

Hey there

Since this issue hasn't had any activity in a while - we're going to automatically close it in 30 days. If you're still seeing this issue with the latest version of Nomad, please respond here and we'll keep this open and take another look at this.

Thanks!

[stale]

This issue will be auto-closed because there hasn't been any activity for a few months. Feel free to open a new one if you still experience this problem 👍

[github-actions]

I'm going to lock this issue because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.