docs(iroh-net): Improve last_pong field docs

This is a bit confusing, at least this description is closer to the
reality.

Not mentioning PING_TIMEOUT_DURATION since that is not strictly true,
if the socket has signs of being alive but a pong is lost it will
still be considered alive and the pong timeout will not be cleared.

iroh-net/src/magicsock/node_map/path_state.rs

@@ -42,7 +42,9 @@ pub(super) struct PathState {
     /// The time this endpoint was last advertised via a call-me-maybe DISCO message.
     pub(super) call_me_maybe_time: Option<Instant>,
 
-    /// Last [`PongReply`] received.
+    /// The most recent [`PongReply`].
+    ///
+    /// Previous replies are cleared when the path is no longer considered alive.
     pub(super) recent_pong: Option<PongReply>,
     /// When the last payload data was **received** via this path.
     ///