Add dummy_thread[ing] sections and reference these from libthread[ing].
diff --git a/Doc/lib/libthreading.tex b/Doc/lib/libthreading.tex
index a152a4d..4961829 100644
--- a/Doc/lib/libthreading.tex
+++ b/Doc/lib/libthreading.tex
@@ -8,6 +8,9 @@
This module constructs higher-level threading interfaces on top of the
lower level \refmodule{thread} module.
+The \refmodule{dummy_threading} module is provided for situations where
+\module{threading} cannot be used because \module{thread} is missing.
+
This module is safe for use with \samp{from threading import *}. It
defines the following functions and objects: