docs: document NO_SIGNAL sentinel and DTR polling interval

src/display/task.rs

@@ -24,7 +24,8 @@ const BOARD_NAME: &str = if cfg!(feature = "rak_wisblock_4631") {
 /// Duration per sparkline slot. 64 slots * 1s = ~1 minute of history.
 const SPARK_SLOT_MS: u64 = 1000;
 
-/// Sentinel: no packet received in this slot.
+/// Sentinel: no packet received in this slot. Below SX1262 sensitivity
+/// floor (-120 dBm), so it cannot be confused with a real RSSI value.
 const NO_SIGNAL: i16 = -121;
 
 struct DisplayState {

src/usb/task.rs

@@ -105,7 +105,7 @@ async fn protocol_loop<'d, D: embassy_usb_driver::Driver<'d>>(
         match select3(
             receiver.read_packet(&mut read_buf),
             responses.receive(),
-            embassy_time::Timer::after_millis(250),
+            embassy_time::Timer::after_millis(250), // DTR poll interval for disconnect detection
         )
         .await
         {