Document the changed fdopen behaviour.
(Hope the markup is ok).
diff --git a/Doc/lib/libos.tex b/Doc/lib/libos.tex
index 2b4728c..fa9f0f3 100644
--- a/Doc/lib/libos.tex
+++ b/Doc/lib/libos.tex
@@ -298,6 +298,10 @@
the corresponding arguments to the built-in \function{open()}
function.
Availability: Macintosh, \UNIX, Windows.
+
+\versionchanged[When specified, the \var{mode} argument must now start
+ with one of the letters \code{'r'}, \code{'w'}, or \code{'a'},
+ otherwise a \exception{ValueError} is raised. ]{2.3}
\end{funcdesc}
\begin{funcdesc}{popen}{command\optional{, mode\optional{, bufsize}}}