clarify allowed property path value types

Author: xabbuh

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

@@ -1,7 +1,7 @@
 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
@@ -10,8 +10,3 @@ 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.