feature #7400 Documented the "labels" option of DateIntervalType (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #7400).

Discussion
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Documented the "labels" option of DateIntervalType

This fixes #7368.

Commits
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ffe2d21 Documented the "labels" option of DateIntervalType

Author: xabbuh

reference/forms/types/dateinterval.rst

@@ -27,6 +27,7 @@ or an array (see `input`_).
 |                      | - `seconds`_                                                                     |
 |                      | - `weeks`_                                                                       |
 |                      | - `input`_                                                                       |
+|                      | - `labels`_                                                                      |
 |                      | - `placeholder`_                                                                 |
 |                      | - `widget`_                                                                      |
 |                      | - `with_days`_                                                                   |
@@ -128,6 +129,28 @@ your underlying object. Valid values are:
 The value that comes back from the form will also be normalized back into
 this format.
 
+labels
+~~~~~~
+
+.. versionadded:: 3.3
+   The ``labels`` option was introduced in Symfony 3.3.
+
+**type**: ``array`` **default**: (see below)
+
+The labels displayed for each of the elements of this type. The default values
+are ``null``, so they display the "humanized version" of the child names (``Invert``,
+``Years``, etc.)::
+
+  'labels' => array(
+      'invert' => null,
+      'years' => null,
+      'months' => null,
+      'days' => null,
+      'hours' => null,
+      'minutes' => null,
+      'seconds' => null,
+  )
+
 minutes
 ~~~~~~~