Consider enabling deadlock_detection in docs.rs builds

[Jake Goulding]

I wanted to see how the deadlock_detection feature worked, so I read the README here, which points to the feature flag. I then popped over to docs.rs and searched for deadlock and found no results. Because of that, I assumed that a deadlock would just result in a panic (which would be reasonable for debugging purposes, IMO). I enabled the feature and caused a deadlock and then... nothing.

Only after building the docs myself with the flag on did I find the parking_lot::deadlock::check_deadlock function and the related module documentation.

When possible, I'd encourage building the docs.rs docs with as many features enabled as possible to help discovery.

I did see that this feature is mutually exclusive (heh) with other features. In those cases, it could be worth having a placeholder module / function that just contains the documentation but not the implementations.