assistant: Make including warnings for the diagnostic command more discoverable (#13323)

This makes the `--include-warnings` argument more discoverable

<img width="651" alt="image"
src="https://github.com/zed-industries/zed/assets/53836821/02d32496-ad7a-4670-85ce-eda945c0f1c2">

Release Notes:

- N/A

crates/assistant/src/slash_command/diagnostics_command.rs

@@ -1,4 +1,4 @@
-use super::{SlashCommand, SlashCommandOutput};
+use super::{create_label_for_command, SlashCommand, SlashCommandOutput};
 use anyhow::{anyhow, Result};
 use assistant_slash_command::SlashCommandOutputSection;
 use fuzzy::{PathMatch, StringMatchCandidate};
@@ -85,6 +85,10 @@ impl SlashCommand for DiagnosticsCommand {
         "diagnostics".into()
     }
 
+    fn label(&self, cx: &AppContext) -> language::CodeLabel {
+        create_label_for_command("diagnostics", &[INCLUDE_WARNINGS_ARGUMENT], cx)
+    }
+
     fn description(&self) -> String {
         "Insert diagnostics".into()
     }