doc: clarify use of .xml or .json extensions in REST interface

Document that /rest and /rest/data do not support extensions.

Author: rouilj

doc/rest.txt

@@ -336,15 +336,17 @@ way to specify alternate acceptable response formats.
 To make testing from the browser easier, you can also append the
 extension ``.json`` or ``.xml`` to the path component of the url. This
 will force json or xml (if supported) output. If you use an extension
-it takes priority over any accept headers.
+it takes priority over any accept headers. Note the extension does not
+work for the ``/rest`` or ``/rest/data`` paths. In these cases it
+returs a 404 error. Adding the header ``Accept: application/xml``
+allows these paths to return xml data.
 
 The rest interface returns status 406 if you use an unrecognized
 extension.  You will also get a 406 status if none of the entries in
 the accept header are available or if the accept header is invalid.
 
 Note: ``dicttoxml2.py`` is an updated version of ``dicttoxml.py``. If
-you are still using Python 2.7 or 3.6, you should use
-``dicttoxml.py``.
+you are still using Python 2.7 or 3.6, you can use ``dicttoxml.py``.
 
 
 General Guidelines