Add ref to errno module.
diff --git a/Doc/lib/libposix.tex b/Doc/lib/libposix.tex
index 131e6e0..803cbbc 100644
--- a/Doc/lib/libposix.tex
+++ b/Doc/lib/libposix.tex
@@ -58,6 +58,8 @@
pair containing the numeric error code from \code{errno} and the
corresponding string, as would be printed by the C function
\code{perror()}.
+See the module \module{errno}\refbimodindex{errno}, which contains
+names for the error codes defined by the underlying operating system.
\end{excdesc}
It defines the following functions and constants: