[Form] Document CollectionType's prototype_data

Author: kgilden

reference/forms/types/collection.rst

@@ -18,6 +18,7 @@ forms, which is useful when creating forms that expose one-to-many relationships
 |             | - `delete_empty`_                                                           |
 |             | - `options`_                                                                |
 |             | - `prototype`_                                                              |
+|             | - `prototype_data`_                                                         |
 |             | - `prototype_name`_                                                         |
 |             | - `type`_                                                                   |
 +-------------+-----------------------------------------------------------------------------+
@@ -330,6 +331,14 @@ rendering your form, having the entire "form row" may be easier for you.
 For details on how to actually use this option, see the above example as well
 as :ref:`cookbook-form-collections-new-prototype`.
 
+prototype_data
+~~~~~~~~~~~~~~
+
+**type**: ``mixed`` **default**: ``null``
+
+Allows you to specify data for the prototype. Each new row added will initially
+contain the data set by this option.
+
 prototype_name
 ~~~~~~~~~~~~~~