Document clearly that the only way to retrieve the return code from the
child processes is to use the Popen3 and Popen4 classes.
This fixes SF bug #460512.
diff --git a/Doc/lib/libos.tex b/Doc/lib/libos.tex
index 16f3690..1c3a584 100644
--- a/Doc/lib/libos.tex
+++ b/Doc/lib/libos.tex
@@ -315,6 +315,12 @@
objects should be opened in binary or text mode. The default value
for \var{mode} is \code{'t'}.
+These methods do not make it possible to retrieve the return code from
+the child processes. The only way to control the input and output
+streams and also retrieve the return codes is to use the
+\class{Popen3} and \class{Popen4} classes from the \refmodule{popen2}
+module; these are only available on \UNIX.
+
\begin{funcdesc}{popen2}{cmd\optional{, mode\optional{, bufsize}}}
Executes \var{cmd} as a sub-process. Returns the file objects
\code{(\var{child_stdin}, \var{child_stdout})}.