[css-cascade-6] Clarify circular dependency (In wording) in finding scoping limits.

[dshin-moz]

Finding scoping limits mention in-scope elements, which in turn mentions scoping limits, which in turn etc etc. Just a wording issue, since it can be inferred that the scoping limit portion is ignored at this time.

Still, I think this is helps reduce confusion on first-read.

[tabatkins]

I inlined the desired half of the definition instead.

Looking over it with fresh eyes, it seemed that "in scope" was probably also the wrong definition anyway, since it includes the scoping root, and I don't think we want scoping limits to be able to match the scoping root itself? That would be easy to hit if you're trying to style a nestable component with @scope (.widget) to (.widget), and would produce a scope with a single element in it, which doesn't seem particularly useful. In the rare case you actually do just want one element like that, just using @scope (.foo) { :scope {...}} works just fine, with no need for a limit.

So as part of inlining it, I also changed "inclusive descendant of the scoping root" to just "descendant of the scoping root".