[Form] Add keep_as_list option

Author: alexandre-daubois

reference/forms/types/collection.rst

@@ -229,6 +229,23 @@ you'd use the :doc:`EmailType </reference/forms/types/email>`. If you want
 to embed a collection of some other form, pass the form type class as this
 option (e.g. ``MyFormType::class``).
 
+keep_as_list
+~~~~~~~~~~~~
+
+**type**: ``boolean`` **default**: ``false``
+
+If set to ``true``, the entries will be reindexed so their indexes follow each
+other. This is useful when you validate a collection type and an item was
+removed from the collection on submit. If you don't reindex the entries, the
+indexes may have gaps in them. For example, if you have a collection of 3
+items and you remove the second item, the indexes will be ``0`` and ``2`` when
+validating the collection. If you reindex the entries by setting the option
+to ``true``, the indexes will be ``0`` and ``1``.
+
+.. versionadded:: 7.1
+
+    The ``keep_as_list`` option was introduced in Symfony 7.1.
+
 prototype
 ~~~~~~~~~