agent: Refine naming for the panel default_view setting (#31446)

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

docs/src/configuring-zed.md

@@ -3284,6 +3284,7 @@ Run the `theme selector: toggle` action in the command palette to see a current
   "dock": "right",
   "default_width": 640,
   "default_height": 320,
+  "default_view": "thread",
   "default_model": {
     "provider": "zed.dev",
     "model": "claude-3-7-sonnet-latest"