Issue #11768: signal.set_wakeup_fd() and PySignal_SetWakeupFd() added in 2.6

Fix the documentation.
diff --git a/Doc/library/signal.rst b/Doc/library/signal.rst
index 74ff78d..9229117 100644
--- a/Doc/library/signal.rst
+++ b/Doc/library/signal.rst
@@ -204,6 +204,8 @@
    attempting to call it from other threads will cause a :exc:`ValueError`
    exception to be raised.
 
+   .. versionadded:: 2.6
+
 
 .. function:: siginterrupt(signalnum, flag)