[2.7] Mention how to disable signal fd wakeup (GH-2140) (#2151)
(cherry picked from commit d79c1d4a9406384f10a37f26a7515ce79f9fdd78)
diff --git a/Doc/library/signal.rst b/Doc/library/signal.rst
index fe7983b..3b02a39 100644
--- a/Doc/library/signal.rst
+++ b/Doc/library/signal.rst
@@ -197,8 +197,10 @@
written to the fd. This can be used by 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.
When threads are enabled, this function can only be called from the main thread;
attempting to call it from other threads will cause a :exc:`ValueError`