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

crates/agent/src/agent_panel.rs

@@ -525,8 +525,8 @@ impl AgentPanel {
 
         let panel_type = AssistantSettings::get_global(cx).default_view;
         let active_view = match panel_type {
-            DefaultView::Agent => ActiveView::thread(thread.clone(), window, cx),
-            DefaultView::Thread => {
+            DefaultView::Thread => ActiveView::thread(thread.clone(), window, cx),
+            DefaultView::TextThread => {
                 let context =
                     context_store.update(cx, |context_store, cx| context_store.create(cx));
                 let lsp_adapter_delegate = make_lsp_adapter_delegate(&project.clone(), cx).unwrap();