Add ref to errno module, and change ref to posix.error to os.error.
diff --git a/Doc/lib/libsocket.tex b/Doc/lib/libsocket.tex
index bede8a8..41b27f9 100644
--- a/Doc/lib/libsocket.tex
+++ b/Doc/lib/libsocket.tex
@@ -53,7 +53,9 @@
The accompanying value is either a string telling what went wrong or a
pair \code{(\var{errno}, \var{string})}
representing an error returned by a system
-call, similar to the value accompanying \code{posix.error}.
+call, similar to the value accompanying \code{os.error}.
+See the module \module{errno}\refbimodindex{errno}, which contains
+names for the error codes defined by the underlying operating system.
\end{excdesc}
\begin{datadesc}{AF_UNIX}