[3.5] Mention how to disable signal fd wakeup (GH-2140) (#2150)

(cherry picked from commit d79c1d4a9406384f10a37f26a7515ce79f9fdd78)

diff --git a/Doc/library/signal.rst b/Doc/library/signal.rst
index 039b666475a247556b3054b765e72dd69662bca2..46d71def08abd6c62745b3b365d4822da515af85 100644 (file)
--- a/Doc/library/signal.rst
+++ b/Doc/library/signal.rst
@@ -306,8 +306,10 @@ The :mod:`signal` module defines the following functions:
    a library to wakeup a poll or select call, allowing the signal to be fully
    processed.
 
-   The old wakeup fd is returned.  *fd* must be non-blocking.  It is up to the
-   library to remove any bytes before calling poll or select again.
+   The old wakeup fd is returned (or -1 if file descriptor wakeup was not
+   enabled).  If *fd* is -1, file descriptor wakeup is disabled.
+   If not -1, *fd* must be non-blocking.  It is up to the library to remove
+   any bytes from *fd* before calling poll or select again.
 
    Use for example ``struct.unpack('%uB' % len(data), data)`` to decode the
    signal numbers list.