See
http://wiki.hudson-ci.org/display/HUDSON/Jelly+form+controls
for the reference.
Server-side method that handles the validation. For example, if this is 'foo', you need "doFoo" on
your descriptor class.
Caption of the validate button. Should be internationalized.
Caption of the text shown while the AJAX call is in progress. For example, "checking..."
','-separated list of fields that are sent to the server.
User-defined url for AJAX request. Default value is ${rootURL}/descriptor/${descriptor.clazz.name}
Additional query params for AJAX request
${attrs.progress}