Move section 4.7 "Natural Language Values" out of "Model"

[evanp]

Most of the subsections of section 4 describe the object model of Activity Streams: Object, Link, Actor, Collection, Activity.

"Natural Language Values" is not a type. Maybe we should just move it to its own top-level section.

[evanp]

It might also be good in section 2.

[evanp]

I think we should move this to section 2.

[TallTed]

Sections are always moving.

Please refer to them by their title or other less-frequently changing identifier.