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

[STINNER Victor]

STINNER Victor added the comment:

> This issue is an EASY issue: please leave it to real newcomers who didn't write any PR to CPython previously. Core developers: explain how to fix the issue, don't fix it ;-)

Oh wait, it seems like the core developer Antoine Pitrou fixed the issue before I created it! => commit d79c1d4a9406384f10a37f26a7515ce79f9fdd78

He already created 3 PR to backport the doc enhancement to 2.7, 3.5 and 3.6 branches.

Sorry!

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<http://bugs.python.org/issue30644>
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