Make slash commands more discoverable (#12480)

<img width="648" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/a63df904-fbbe-4e0a-80b2-c98ebee90690">

Release Notes:

- N/A

Co-authored-by: Nathan <nathan@zed.dev>

crates/assistant_slash_command/src/assistant_slash_command.rs

@@ -20,7 +20,7 @@ pub trait SlashCommand: 'static + Send + Sync {
         CodeLabel::plain(self.name(), None)
     }
     fn description(&self) -> String;
-    fn tooltip_text(&self) -> String;
+    fn menu_text(&self) -> String;
     fn complete_argument(
         &self,
         query: String,