add comment for uart transceiver power

Author: hathach

cores/nRF5/Uart.cpp

@@ -163,6 +163,8 @@ void Uart::end()
   nrfUart->TASKS_STOPTX = 0x1UL;
 
   // Wait for TXSTOPPED event and for RXTO event
+  // This is required before disabling UART to fully power down transceiver PHY.
+  // Otherwise transceiver will continue to consume ~900uA
   while ( !(nrfUart->EVENTS_TXSTOPPED && nrfUart->EVENTS_RXTO) ) yield();
 
   nrfUart->ENABLE = UARTE_ENABLE_ENABLE_Disabled;