Mention how to disable signal fd wakeup (python#2140)

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.