Simplifying SCID and XVID descriptions

[wenjing]

(3) SCID and XVID
These are new and a good way to classify VIDs, but the wording is a little too complex and tied to specifics. If OK with everyone, I'd write something more abstract. We really just need to say one type is self-contained, the other needs an external entity. Then we list an example or two. I'll propose some text.