How to move "description" below the "label" for a field in a form?
Have a look at Label help module. It just creates a second help field that will appear directly below the form element's label.

It is a simple way to turn around the problem.

Or try something like 

jQuery(document).ready(function ($) {

  // move the help text from below the input field to directly below the field label

  $('.form-item .description').each(function () {  // description is the help text
    var desc = $(this);
    var label = desc.siblings('label:first');
     if (label.length) {
       desc.insertAfter(label);
     }
  })
  // the help text is tangled up in the text format stuff on a filtered text field
  $('.text-format-wrapper .description').each(function () {
    var desc = $(this);
    var label = desc.siblings('.form-item').find('label:first');
     if (label.length) {
        desc.insertAfter(label);
     }
  })
});

(thanks to El bandito !)