Doc: Add DateTimePicker documentation

refs #4547

doc/form_elements.md

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+# Form Elements Shipped With Icinga 2 Web
+
+On top of the elements provided by the Zend Framework, Icinga 2 Web ships its own to offer additional functionality.
+The following is a list of these classes, as well as descriptions of the functionality they offer.
+
+## DateTimePicker
+
+`Icinga\Web\Form\Element\DateTimePicker` represents a control that allows the user to select date/time and to
+display the date and time with a user specified format. Internally the element returns the input as Unix timestamp after
+it has been proven valid. That is when the input is valid to `\DateTime::createFromFormat()` according to the user
+specified format. Input is always timezone aware because the element utilizes `Icinga\Util\DateTimeFactory` which relies
+on the timezone set by the user.
+
+**Example #1 DateTimePicker expecting date**
+
+    use Icinga\Web\Form\Element\DateTimePicker;
+
+    $element = new DateTimePicker(
+        array(
+            'name'      => 'date',
+            'label'     => t('Date'),
+            'patterns'  => array('Y-m-d') // Allowed format
+        )
+    )
+
+**Example #2 DateTimePicker expecting time**
+
+    use Icinga\Web\Form\Element\DateTimePicker;
+
+    $element = new DateTimePicker(
+        array(
+            'name'      => 'time',
+            'label'     => t('Time'),
+            'patterns'  => array('H:i:s') // Allowed format
+        )
+    )
+
+**Example #3 DateTimePicker expecting date and time**
+
+    use Icinga\Web\Form\Element\DateTimePicker;
+
+    $element = new DateTimePicker(
+        array(
+            'name'      => 'datetime',
+            'label'     => t('Date And Time'),
+            'patterns'  => array('Y-m-d H:i:s') // Allowed format
+        )
+    )
+
+**Example #4 DateTimePicker expecting date/time w/ default value**
+
+    use Icinga\Web\Form\Element\DateTimePicker;
+    use Icinga\Util\DateTimeFactory;
+
+    $now = DateTimeFactory::create();
+
+    $element =  new DateTimePicker(
+        array(
+            'name'      => 'datetime',
+            'label'     => t('Date/Time'),
+            'value'     => $now->getTimestamp() + 3600, // now plus 1 hour
+            'patterns'  => array('Y-m-d H:i:s', 'Y-m-d', 'H:i:s') // Allowed format
+        )
+    )
+
+