added the possibility to define a form date format as a string

Author: fabpot

reference/forms/types/options/date_format.rst.inc

@@ -1,4 +1,6 @@
-*   ``format`` [type: integer, default: IntlDateFormatter::MEDIUM]
-    Option passed to the IntlDateFormatter class, used to transform user input
+*   ``format`` [type: integer or string, default: ``IntlDateFormatter::MEDIUM``]
+    Option passed to the ``IntlDateFormatter`` class, used to transform user input
     into the proper format. This is critical when the ``widget`` option is
-    set to ``text``, and will define how to transform the input.
+    set to ``text``, and will define how to transform the input. By default,
+    the format is determined based on the current user locale; you can override
+    it by passing the format as a string.