Document that destructors in running threads are not run on program exit

[jwodder]

It's my understanding that, when a Rust program's main thread terminates and thereby causes any still-running threads to be shut down, destructors are not run for the shut-down threads; however, the documentation doesn't seem to come out and say that explicitly anywhere. This PR therefore adds an explicit statement about this to the std::thread docs.

If I am mistaken and destructors are run, that's worth documenting explicitly, too.

[rustbot]

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