Improve the documentation for #ajax['trigger_as']

This setting was added by #684846: AJAX triggered by non-submit element fails if any elements are validated → . It is an array that provides a 'name' and an optional 'value' (for the case when different buttons have the same 'name', such as "op" or "submit"). These should be the same #name and #value info the of the element that triggers the form submission. #name encodes the parents of the element in it, the same as form_set_error(), i.e. "level1][level2][element", though this is dependent on the value of #tree.