Update input option for MoneyType

Author: StevenRenaux

reference/forms/types/money.rst

@@ -83,6 +83,9 @@ input
 By default, the money value is converted to a ``float`` PHP type. If you need the
 value to be converted into an integer (e.g. because some library needs money
 values stored in cents as integers) set this option to ``integer``.
+You can also set this option to ``string``, it can be useful if the underlying
+data is a string for precision reasons (for example, Doctrine uses strings for
+the decimal type).
 
 .. versionadded:: 7.1