Mention how to disable signal fd wakeup (#2140)
diff --git a/Doc/library/signal.rst b/Doc/library/signal.rst
index 039b666..46d71de 100644
--- a/Doc/library/signal.rst
+++ b/Doc/library/signal.rst
@@ -306,8 +306,10 @@
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.