update documentation for mass input js util

static/js/utils/massInput.js

@@ -5,14 +5,19 @@
    *
    * Mass Input Utility
    *  allows form shapes to be manipulated asynchronously:
-   *  will asynchronously submit the form and replace the contents of the
-   *  mass input with the one from the BE response
+   *  will asynchronously submit the containing form and replace the contents
+   *  of the mass input element with the one from the BE response
+   *  The utility will only trigger an AJAX request if the mass input element has
+   *  an active/focused element whilst the form is being submitted.
    *
    * Attribute:   uw-mass-input
-   *   (will be set up automatically on tables)
    *
    * Example usage:
-   *  (table with one column thats only checkboxes)
+   *  <form method="POST" action="...">
+   *   <input type="text">
+   *   <div uw-mass-input>
+   *     <input type="text">
+   *     <button type="submit">
    */
 
   var MASS_INPUT_UTIL_NAME = 'massInput';