doc/texipod: add rule to correctly interpret @ref{ANCHOR,XREF,SECTION_NAME,...}

This allows to name an internal reference in the POD/MAN output.

doc/texi2pod.pl

@@ -361,6 +361,7 @@ sub postprocess
     s/\(?\@xref\{(?:[^\}]*)\}(?:[^.<]|(?:<[^<>]*>))*\.\)?//g;
     s/\s+\(\@pxref\{(?:[^\}]*)\}\)//g;
     s/;\s+\@pxref\{(?:[^\}]*)\}//g;
+    s/\@ref\{(?:[^,]*,)(?:[^,]*,)([^,\}]*).*\}/$1/g;
     s/\@ref\{([^\}]*)\}/$1/g;
     s/\@noindent\s*//g;
     s/\@refill//g;