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

[STINNER Victor]

New submission from STINNER Victor:

The signal.set_wakeup_fd() function is used to write signal numbers as bytes into a pipe, to be able to handle signals using a "regular" file descriptors instead of special signal handlers.

https://docs.python.org/dev/library/signal.html

Problem: it's not documented how to unregister the FD. I just saw "signal.set_wakeup_fd(-1)" in a Pull Request and realized that we forgot to document it:
https://github.com/python/cpython/pull/2139/

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 ;-)

Doc/library/signal.rst should be modified. I read that it's even possible to create a PR using an online editor? I never tried:
https://github.com/python/cpython/blob/master/Doc/library/signal.rst

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