Merge branch '4.2'

* 4.2:
  Show example on using a DateTimeImmutable object
  Describe actual traversal behaviour

Author: javiereguiluz

reference/constraints/Valid.rst

@@ -264,6 +264,6 @@ traverse
 
 **type**: ``boolean`` **default**: ``true``
 
-If this constraint is applied to a property that holds an array of objects,
-then each object in that array will be validated only if this option is
-set to ``true``.
+If this constraint is applied to a ``Traversable``, then all containing values
+will be validated if this option is set to ``true``. This option is ignored on
+arrays: Arrays are traversed in either case. Keys are not validated.

reference/forms/types/date.rst

@@ -67,8 +67,13 @@ field as **three different choice fields**::
         'widget' => 'choice',
     ]);
 
-If your underlying date is *not* a ``DateTime`` object (e.g. it's a unix timestamp),
-configure the `input`_ option.
+If your underlying date is *not* a ``DateTime`` object (e.g. it's a unix
+timestamp or a ``DateTimeImmutable`` object), configure the `input`_ option::
+
+    $builder->add('publishedAt', DateType::class, [
+        'widget' => 'choice',
+        'input'  => 'datetime_immutable'
+    ]);
 
 Rendering a single HTML5 Textbox
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~