Documentation for spock.util.concurrent classes

[Vampire]

The classes in package spock.util.concurrent are only briefly mentioned in the Changelog.
Please add documentation for them to the normal Userguide.

[MykolaBova]

Hi! Is this issue still actual?
If yes Can you provide some technical details what and how supposed to be improved?

[kriegaex]

This issue overlaps with the much older #633.

The delta is that the other one also mentions Hamcrest expect/closeTo. While the Spock manual does not need to copy the Hamcrest documentation, I think it would make sense to add a small example for how to use those parts which Spock has special support for. So far, they are only briefly mentioned in release notes. @thokari, would you like to add that to #1173, too? Or does it make more sense to open a separate PR for that? It might also depend on the reviewers' preferences.

Update: Oh, OK, there was just a new comment on the old PR. I thought the PR was more recent. It seems as if there are review issues. I have not looked into it at all, my bad. But at least I am aware of this issue now, kind of duplicating mine.