assistant: Indicate when the /rustdoc output is from the index (#13148)

This PR makes it so that when `/rustdoc` returns content from the local
index it indicates as such in the placeholder.

Release Notes:

- N/A

crates/rustdoc/src/indexer.rs

@@ -14,6 +14,8 @@ use crate::{
 
 #[derive(Debug, Clone, Copy)]
 pub enum RustdocSource {
+    /// The docs were sourced from Zed's rustdoc index.
+    Index,
     /// The docs were sourced from local `cargo doc` output.
     Local,
     /// The docs were sourced from `docs.rs`.