Mentioning data-index too

Author: ThomasLandauer

form/form_collections.rst

@@ -238,8 +238,9 @@ it will receive an *unknown* number of tags. Otherwise, you'll see a
 
 The ``allow_add`` option also makes a ``prototype`` variable available to you.
 This "prototype" is a little "template" that contains all the HTML needed to
-dynamically create any new "tag" forms with JavaScript. To render the prototype, add
-the following ``data-prototype`` attribute to the existing ``<ul>`` in your template:
+dynamically create any new "tag" forms with JavaScript. Now add the following
+``data-index`` (containing the current "form row" number for the following JavaScript)
+and ``data-prototype`` attribute to the existing ``<ul>`` in your template:
 
 .. code-block:: html+twig
 
@@ -265,7 +266,7 @@ On the rendered page, the result will look something like this:
 .. tip::
 
     The ``form.tags.vars.prototype`` is a form element that looks and feels just
-    like the individual ``form_widget(tag)`` elements inside your ``for`` loop.
+    like the individual ``form_widget(tag.*)`` elements inside your ``for`` loop.
     This means that you can call ``form_widget()``, ``form_row()`` or ``form_label()``
     on it. You could even choose to render only one of its fields (e.g. the
     ``name`` field):