diff --git a/Doc/lib/libsocket.tex b/Doc/lib/libsocket.tex
index 199b97f..de2f51d 100644
--- a/Doc/lib/libsocket.tex
+++ b/Doc/lib/libsocket.tex
@@ -480,6 +480,10 @@
 \begin{methoddesc}[socket]{fileno}{}
 Return the socket's file descriptor (a small integer).  This is useful
 with \function{select.select()}.
+
+Under Windows the small integer returned by this method cannot be used where
+a file descriptor can be used (such as \function{os.fdopen()}).  \UNIX{} does
+not have this limitation.
 \end{methoddesc}
 
 \begin{methoddesc}[socket]{getpeername}{}