Editorial: clarify interface mixins' elements vs. inherited ones

Author: littledan

index.bs

@@ -1467,6 +1467,15 @@ and [=operations=] describe the behaviors that can be invoked on the object.
 [=Constants=] declare named constant values
 that are exposed as a convenience to users of objects in the system.
 
+<div class="note">
+
+    When an [=interface=] [=includes=] an [=interface mixin=], each [=member=]
+    of the interface mixin is also considered a member of the interface. In contrast,
+    [=interface/inherit|inherited=] interface members are not considered members
+    of the interface.
+
+</div>
+
 [=Interfaces=], [=interface mixins=], and [=namespaces=] each support a different set of [=members=],
 which are specified in [[#idl-interfaces]], [[#idl-interface-mixins]], and [[#idl-namespaces]],
 and summarized in the following informative table: