Document the mapped formType option

Author: Grégoire Paris

reference/forms/types/form.rst

@@ -19,6 +19,8 @@ on all fields.
 
 .. include:: /reference/forms/types/options/trim.rst.inc
 
+.. include:: /reference/forms/types/options/mapped.rst.inc
+
 .. include:: /reference/forms/types/options/property_path.rst.inc
 
 .. include:: /reference/forms/types/options/attr.rst.inc

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

@@ -0,0 +1,7 @@
+property_path
+~~~~~~~~~~~~~
+
+**type**: ``boolean``
+
+If you wish the field to be ignored when reading or writing to the object, you
+can set the ``mapped`` option to ``false``

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

@@ -12,3 +12,6 @@ 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``
+
+.. versionadded:: 2.1
+    Since 2.1, the ``mapped`` option has been added for this use-case