API to introspect semantic meaning of setting a thread pool limit #214

The scope (thread-local / process-wide) is covered by #211.

This issue covers introspecting if setting the number of threads:

1. Is a per-thread limit for the current thread only. This is (probably) always the case if the API scope is current thread only.
2. Assuming the API's scope affects the whole process, is it setting a per-thread limit for the whole process (what MKL does)...
3. ...or, is it changing the size of a process-wide global thread pool (OpenBLAS with pthreads).

#211 has some discussion of how one might approach this.

It seems, apriori, to be more tricker than scope, so perhaps it shouldn't be a user-facing piece of information, but rather just something run in CI to ensure any future docs/annotations match the real semantics.