Add dummy_thread[ing] sections and reference these from libthread[ing].
diff --git a/Doc/lib/libthread.tex b/Doc/lib/libthread.tex
index d302958..6dfd875 100644
--- a/Doc/lib/libthread.tex
+++ b/Doc/lib/libthread.tex
@@ -17,7 +17,9 @@
The module is optional. It is supported on Windows, Linux, SGI
IRIX, Solaris 2.x, as well as on systems that have a \POSIX{} thread
-(a.k.a. ``pthread'') implementation.
+(a.k.a. ``pthread'') implementation. For systems lacking the \module{thread}
+module, the \refmodule{dummy_thread} module is available. It duplicates this
+module's interface and can be used as a drop-in replacement.
\index{pthreads}
\indexii{threads}{\POSIX}