feature #4892 Add possible values for widget_type (XitasoChris)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #4892).

Discussion
----------

Add possible values for widget_type

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

The documentation does not list which values are possible for widget_type. These two are the ones I extracted from the Symfony2 source code.

Commits
-------

e57c2ac terminate file with newline
ef9e1bd move title back and move description to separate file as suggested in PR #4892
6b42685 move title out of included file as suggested in PR #4892
6fcdd89 Add possible values for widget_type

Author: weaverryan

reference/forms/types/datetime.rst

@@ -61,9 +61,7 @@ for more details.
 date_widget
 ~~~~~~~~~~~
 
-**type**: ``string`` **default**: ``choice``
-
-Defines the ``widget`` option for the :doc:`date </reference/forms/types/date>` type
+.. include:: /reference/forms/types/options/date_widget_description.rst.inc
 
 .. include:: /reference/forms/types/options/days.rst.inc
 

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

@@ -1,14 +1,4 @@
 widget
 ~~~~~~
 
-**type**: ``string`` **default**: ``choice``
-
-The basic way in which this field should be rendered. Can be one of the following:
-
-* ``choice``: renders three select inputs. The order of the selects is defined
-  in the `format`_ option.
-
-* ``text``: renders a three field input of type ``text`` (month, day, year).
-
-* ``single_text``: renders a single input of type ``date``. User's input is
-  validated based on the `format`_ option.
+.. include:: /reference/forms/types/options/date_widget_description.rst.inc

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

@@ -0,0 +1,11 @@
+**type**: ``string`` **default**: ``choice``
+
+The basic way in which this field should be rendered. Can be one of the following:
+
+* ``choice``: renders three select inputs. The order of the selects is defined
+  in the `format`_ option.
+
+* ``text``: renders a three field input of type ``text`` (month, day, year).
+
+* ``single_text``: renders a single input of type ``date``. User's input is
+  validated based on the `format`_ option.