Add deprecation documentation to code

[astubbs]

For example, specifically, ConcurrentQueueSpec is marked as deprecated, but there's no information what's been deprecated in favour of, or why. The source commit (614a6f0) also gives no information.

@Deprecated//(since = "3.0.0")
public final class ConcurrentQueueSpec

[nitsanw]

@astubbs Thanks for reporting! can you contribute a PR?

[astubbs]

Normally I'd say sure - but I still have no idea why it's been deprecated! :)

[nitsanw]

Oh, that's because no external usage was reported and we aim to minimize the surface area.