Improve `ephemeral_disk` documentation

### Proposal

Improve the documentation for `ephemeral_disk`. Currently https://www.nomadproject.io/docs/job-specification/ephemeral_disk is sparse, and doesn't allow me to answer the following questions:

* `migrate` explains what happens when `sticky` is `true` - but what does it do if it's `false`?
* `sticky = true` makes sense, but again - what does `sticky = false` mean?
* There are two references to "best-effort", but I don't think this is precise enough to understand the implications. Under what circumstances will these "best-efforts" fail? What is the alternative? Can I lose data?

### Attempted Solutions

As I don't know the answer to these questions, I'm unable to provide a solution

Start with the ephemeral_disk documentation at https://www.nomadproject.io/docs/job-specification/ephemeral_disk and investigate the behavior of migrate, sticky, and the documented best-effort operations. Done means the page clearly explains both sticky settings, migration behavior, failure circumstances, alternatives, and potential data loss.

Written by the indexing model from the issue text.