Auto merge of #24741 - cgwalters:note-setenv-and-unsetenv-are-not-threadsafe, r=alexc...

Auto merge of #24741 - cgwalters:note-setenv-and-unsetenv-are-not-threadsafe, r=alexcrichton

See:
https://sourceware.org/bugzilla/show_bug.cgi?id=4887#c9
https://bugs.freedesktop.org/show_bug.cgi?id=65681

I just noticed this while talking to someone who was using
`os.environ['FOO'] = 'BAR'` in Python and since I'm learning Rust, I
was curious if it did anything special here (and the answer appears to
be no).

Java got this right by disallowing `setenv()` from the start.