[reference][forms] adding more details on date format - closes #914

Author: weaverryan

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

@@ -5,6 +5,17 @@ format
 
 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 ``single_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.
+set to ``single_text``, and will define how the user will input the data.
+By default, the format is determined based on the current user locale; you
+can override it by passing the format as a string.
+
+For more information on valid formats, see `Date/Time Format Syntax`_. For
+example, to render a single text box that expects the user to end ``YYYY-MM-dd``,
+use the following options::
+
+    $builder->add('date_created', 'date', array(
+        'widget' => 'single_text',
+        'format' => 'YYYY-MM-dd',
+    ));
+
+.. _`Date/Time Format Syntax`: http://userguide.icu-project.org/formatparse/datetime#TOC-Date-Time-Format-Syntax