Provide version changed info

Doc/lib/libcmd.tex

@@ -28,6 +28,7 @@ input and output file objects that the Cmd instance or subclass
 instance will use for input and output. If not specified, they
 will default to \var{sys.stdin} and \var{sys.stdout}.
 
+\versionchanged[The \var{stdin} and \var{stdout} parameters were added.]{2.3}
 \end{classdesc}
 
 \subsection{Cmd Objects}