document the input option of the NumberType

Author: xabbuh

reference/forms/types/number.rst

@@ -13,6 +13,7 @@ that you want to use for your number.
 +-------------+----------------------------------------------------------------------+
 | Options     | - `grouping`_                                                        |
 |             | - `html5`_                                                           |
+|             | - `input`_                                                           |
 |             | - `scale`_                                                           |
 |             | - `rounding_mode`_                                                   |
 +-------------+----------------------------------------------------------------------+
@@ -59,6 +60,18 @@ html5
 If set to ``true``, the HTML input will be rendered as a native HTML5 ``type="number"``
 form.
 
+input
+~~~~~
+
+**type**: ``string`` **default**: ``number``
+
+.. versionadded:: 4.3
+
+    The ``input`` option was introduced in Symfony 4.3.
+
+The format of the input data - i.e. the format that the number is stored on
+your underlying object. Valid values are ``number`` and ``string``.
+
 scale
 ~~~~~