Follow up to https://github.com/zed-industries/zed/pull/31353. Just
ensuring we're walking toward a more consistent use of the multiple
terms we have floating around in the AI realm. In this case, `thread` is
the term for the now default view, the one that has agentic features;
`text_thread` is the term for the original view, the one where it's just
text. The settings now reflect this. Also took advantage of the
opportunity to add some docs, too.

Release Notes:

- N/A