[docs] the ThreadPool struct in rayon-core needs documentation

[nikomatsakis]

The ThreadPool struct represents a user created thread-pool (as opposed to the default, global threadpool). It needs /// foo docs.

Some things to say:

• Create a new thread-pool with ThreadPool::new (!). You can use the configuration to specify the number or names of its threads, how to handle uncaptured panics, and so forth.
• There are two ways to use the thread-pool:
  • The first is that you can execute ThreadPool::install(). This will execute a closure in one of the threadpool's threads. Any other Rayon operations -- like parallel iterators, join(), or scope() -- will then execute in the context of this thread-pool.
  • The second is that you invoke ThreadPool::spawn_async() or ThreadPool::spawn_future_async(). These methods will inject jobs into the thread-pool that are not tied to your current stack frame; however, those jobs cannot access data owned by the stack.
• When the thread-pool is dropped, that is a signal for its threads to terminate. Once they have completed executing any remaining work that you have spawned, they will automatically terminate.

[dmfutcher]

Can this be closed given #325 was merged?

[cuviper]

Sounds good, thanks!

[munael]

Are there any non-doc [help wanted] issues remaining or coming?

[cuviper]

@Enamex I would say docs are where we're most lacking, but I'll try to remember to file any other ideas as I have them. I just filed #465, and maybe you could check if someone is still working on #330.

There are also open questions, like how to create parallel iterators from arbitrary iterators (#46) or vice versa (#210). I guess those could be considered [help wanted] in the sense of "help, we don't even know how to do this!" We tend to only tag [help wanted] on mentor-able issues, but anyone is welcome to chime in on other issues too.