Document thread_local behavior with foreign-created threads

Location

https://doc.rust-lang.org/std/macro.thread_local.html
https://doc.rust-lang.org/std/thread/struct.LocalKey.html

Summary

Rust can be used as a static or dynamic library in a host process in another language/runtime. The thread_local documentation makes no mention about how foreign threads behave with respect to to thread locals created via LocalKey or the !thread_local macro.

I think it is safe, with one possible exception. I think the const {} syntax allows Rust to avoid lazy initialization and instead use a more efficient implementation that may rely on statically allocated thread-local storage (TLS) instead of dynamically allocated TLS keys. I do not see how this would work with foreign-created threads that call into Rust.

Researching this has proved difficult, I think the docs should clarify the behavior and what is guaranteed and what isn't when it comes to foreign-created threads.

Start with the documentation at std::thread_local and std:🧵:LocalKey, then investigate the behavior of foreign-created threads calling into Rust. Document what is guaranteed for LocalKey and the thread_local! macro, including the const {} case and any limitations, so the two linked pages clearly explain the supported behavior.