nit: state machine desc (#16569)

codex-rs/core/src/state/turn.rs

@@ -30,6 +30,16 @@ pub(crate) struct ActiveTurn {
 }
 
 /// Whether mailbox deliveries should still be folded into the current turn.
+///
+/// State machine:
+/// - A turn starts in `CurrentTurn`, so queued child mail can join the next
+///   model request for that turn.
+/// - After user-visible terminal output is recorded, we switch to `NextTurn`
+///   to leave late child mail queued instead of extending an already shown
+///   answer.
+/// - If the same task later gets explicit same-turn work again (a steered user
+///   prompt or a tool call after an untagged preamble), we reopen `CurrentTurn`
+///   so that pending child mail is drained into that follow-up request.
 #[derive(Clone, Copy, Debug, Default, Eq, PartialEq)]
 pub(crate) enum MailboxDeliveryPhase {
     /// Incoming mailbox messages can still be consumed by the current turn.