[docs] [issue30644] [EASY][doc] Document that signal.set_wakeup_fd(-1), unregister the previsouly set wakeup FD

[Antoine Pitrou]

Changes by Antoine Pitrou <pitrou at free.fr>:


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pull_requests: +2201

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30644>
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