Improved the explanation about the "compoun" option

Author: javiereguiluz

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

@@ -3,6 +3,6 @@ compound
 
 **type**: ``boolean`` **default**: ``false``
 
-This option specifies if the type is compound. This is independent of whether
-the type actually has children. A type can be compound but not have any
-children at all.
+This option specifies whether the type contains child types or not. This option
+is managed internally for built-in types, so there is no need to configure
+it explicitly.