[xml] Move to "Experimental" target language status

The XML target language support is not in good shape and is likely to be
removed unless somebody steps up to bring it up to the expected standard
(it fails to even meet the criteria for "Experimental" currently).

Closes #2213

Doc/Manual/SWIG.html

@@ -134,11 +134,11 @@ Supported Target Language Options
      -ruby           - Generate Ruby wrappers
      -scilab         - Generate Scilab wrappers
      -tcl8           - Generate Tcl 8 wrappers
-     -xml            - Generate XML wrappers
 
 Experimental Target Language Options
      -mzscheme       - Generate MzScheme/Racket wrappers
      -ocaml          - Generate OCaml wrappers
+     -xml            - Generate XML wrappers
 
 General Options
      -addextern      - Add extra extern declarations