At David Ascher's request, clarify that os.system() always returns 0

on Win95 & Win98.

Doc/lib/libos.tex

@@ -694,7 +694,8 @@ calling the Standard C function \cfunction{system()}, and has the
 same limitations.  Changes to \code{posix.environ}, \code{sys.stdin},
 etc.\ are not reflected in the environment of the executed command.
 The return value is the exit status of the process encoded in the
-format specified for \function{wait()}.
+format specified for \function{wait()}, except on Windows 95 and 98,
+where it is always \code{0}.
 Availability: \UNIX{}, Windows.
 \end{funcdesc}