include/qemu/thread.h: Document qemu_thread_atexit* API

Add documentation for the qemu_thread_atexit_add() and
qemu_thread_atexit_remove() functions.

We include a (previously undocumented) constraint that notifiers
may not be called if a thread is exiting because the entire
process is exiting. This is fine for our current use because
the callers use it only for cleaning up resources which go away
on process exit (memory, Win32 fibers), and we will need the
flexibility for the new posix implementation.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181105135538.28025-2-peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>