Enum values should be valid to their schema (#212)

guide/src/main/asciidoc/api_specifications.adoc

@@ -457,6 +457,8 @@ A fixed list of possible values of a property can be specified using `enum`.
 However, this may make it harder to change the list of possible values, as client applications will often depend on the specified list e.g. by using code generation.
 
 `enum` SHOULD only be used when the list of values is unlikely to change or when changing it has a big impact on clients of the API.
+
+`enum` values SHOULD be valid to the schema in which they are defined.
 ====
 
 .Enum declaration