Clarify uart_write_blocking() API vs UART still busy sending (#1274) (#1709)

src/rp2_common/hardware_uart/include/hardware/uart.h

@@ -312,6 +312,9 @@ static inline bool uart_is_readable(uart_inst_t *uart) {
  *  \ingroup hardware_uart
  *
  * This function will block until all the data has been sent to the UART transmit buffer
+ * hardware. Note: Serial data transmission will continue until the Tx FIFO and
+ * the transmit shift register (not programmer-accessible) are empty.
+ * To ensure the UART FIFO has been emptied, you can use \ref uart_tx_wait_blocking()
  *
  * \param uart UART instance. \ref uart0 or \ref uart1
  * \param src The bytes to send