Revert "[#3338] Removing delete_empty details because this is first b…

…eing committed to 2.3"

This re-introduces the delete_empty documentation to the master (2.5) branch, since it should live there.

This reverts commit 72fa7b4.

reference/forms/types/collection.rst

@@ -19,6 +19,7 @@ forms, which is useful when creating forms that expose one-to-many relationships
 |             | - `allow_delete`_                                                           |
 |             | - `prototype`_                                                              |
 |             | - `prototype_name`_                                                         |
+|             | - `delete_empty`_                                                           |
 +-------------+-----------------------------------------------------------------------------+
 | Inherited   | - `label`_                                                                  |
 |             | - `label_attr`_                                                             |
@@ -335,6 +336,16 @@ If you have several collections in your form, or worse, nested collections
 you may want to change the placeholder so that unrelated placeholders are not
 replaced with the same value.
 
+delete_empty
+~~~~~~~~~~~~
+
+**type**: ``Boolean`` **default**: ``false``
+
+If you want to explicitly remove entirely empty collection entries from your
+form you have to set this option to true. However, existing collection entries
+will only be deleted if you have the allow_delete_ option enabled. Otherwise
+the empty values will be kept.
+
 Inherited options
 -----------------