[Form] Document CollectionType's prototype_data

Author: kgilden

reference/forms/types/collection.rst

@@ -20,6 +20,7 @@ photos).
 | | - `entry_options`_ |
 | | - `entry_type`_ |
 | | - `prototype`_ |
+| | - `prototype_data`_ |
 | | - `prototype_name`_ |
 +-------------+-----------------------------------------------------------------------------+
 | Inherited | - `by_reference`_ |
@@ -363,6 +364,14 @@ 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
 ~~~~~~~~~~~~~~