clarify allowed property path value types

Author: xabbuh

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

@@ -1,17 +1,12 @@
 property_path
 ~~~~~~~~~~~~~
 
-**type**: ``any`` **default**: ``the field's name``
+**type**: ``PropertyPathInterface|string|null`` **default**: ``null``
 
 Fields display a property value of the form's domain object by default.
 When the form is submitted, the submitted value is written back into the
 object.
 
 If you want to override the property that a field reads from and writes
-to, you can set the ``property_path`` option. Its default value is the field's
-name.
-
-If you wish the field to be ignored when reading or writing to the object
-you can set the ``property_path`` option to ``false``, but using
-``property_path`` for this purpose is deprecated, you should use the
-``mapped`` option.
+to, you can set the ``property_path`` option. Its default value (``null``)
+will use the field's name as the property.