[specification] Commentary contains wrong statement about inheritance

The language specification says

> A class does not inherit members from its superinterfaces.
> However, its implicit interface does.

which is wrong. It should say 'from its non-superclass superinterfaces', and it should probably say 'class or mixin class' etc.

Locate the quoted inheritance statement in the language specification and read the surrounding definitions for classes, mixin classes, and superinterfaces. Update the statement to reflect the issue's requested distinction and terminology, then verify that the surrounding specification remains consistent.

Written by the indexing model from the issue text.