thread::scope: document how join interacts with TLS destructors
Fixes https://github.com/rust-lang/rust/issues/116237 by documenting the current behavior regarding thread-local destructors as intended. (I'm not stoked about this, but documenting it is better than leaving it unclear.)
This also adds documentation for explicit `join` calls (both for scoped and regular threads), saying that those *will* wait for TLS destructors. That reflects my understanding of the current implementation, which calls `join` on the native thread handle. Are we okay with guaranteeing that? I think we should, so people have at least some chance of implementing "wait for all destructors" manually. This fixes https://github.com/rust-lang/rust/issues/127571.
Cc @rust-lang/libs-api
faa0d67374