export maxthreadid from Threads

[KristofferC]

maxthreadid is in the documentation and is referred to by other public docstrings so it should at least itself be public. Looking at what else is being exported I think this has the same "weight".

[fingolfin]

It is mentioned in the Threads.nthreads docs, and also listed in doc/src/base/multi-threading.md so this seems reasonable?

@vtjnash reacted with a 👎 perhaps you could elaborate what your concerns are?

[vtjnash]

There are almost no uses of this outside of internal scheduler code that are not subtle races and UB if you access this value

[fingolfin]

OK, I find it a convincing argument that this should not be merged.

But then any docstrings of exported/public bindings referencing maxthreadid should likely also be adjusted to not do that anymore.

The only docstring I found referencing maxthreadid (besides its own docstring) is that of Threads.nthreads, but there it seems straight-forward to excise it... Did I miss anything?