tui: clarify live slash queueing branch

Document that the busy-path queueing in dispatch_command is only reached during live slash dispatch, not queued replay, so the returned drain-control value is effectively a throwaway for that branch.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/tui/src/chatwidget.rs

@@ -4323,6 +4323,9 @@ impl ChatWidget {
             && !matches!(cmd, SlashCommand::Model | SlashCommand::Review)
         {
             self.queue_user_message(format!("/{}", cmd.command()).into());
+            // This busy-path queueing only happens for live command dispatch. Queued replay
+            // executes slash drafts only while idle, and handle_serialized_slash_command() queues
+            // instead of dispatching when a task is already running.
             return QueueReplayControl::Stop;
         }
         match cmd {