docs/library/machine.UART: Add docs for uart.flush() and uart.txdone().

docs/library/machine.UART.rst

@@ -177,6 +177,32 @@ Methods
 
    Availability: WiPy.
 
+.. method:: UART.flush()
+
+   Waits until all data has been sent. In case of a timeout, an exception is raised. The timeout
+   duration depends on the tx buffer size and the baud rate. Unless flow control is enabled, a timeout
+   should not occur.
+
+   .. note::
+
+       For the rp2, esp8266 and nrf ports the call returns while the last byte is sent.
+       If required, a one character wait time has to be added in the calling script.
+
+   Availability: rp2, esp32, esp8266, mimxrt, cc3200, stm32, nrf ports
+
+.. method:: UART.txdone()
+
+   Tells whether all data has been sent or no data transfer is happening. In this case,
+   it returns ``True``. If a data transmission is ongoing it returns ``False``.
+
+   .. note::
+
+       For the rp2, esp8266 and nrf ports the call may return ``True`` even if the last byte
+       of a transfer is still being sent. If required, a one character wait time has to be
+       added in the calling script.
+
+   Availability: rp2, esp32, esp8266, mimxrt, cc3200, stm32, nrf ports
+
 Constants
 ---------