docs/library/machine.UART: Add notes about UART init and deinit.

* `init()` can be called multiple times to reconfigure UART.
* After `deinit()` it is impossible to call `init()` again.

Signed-off-by: Tomasz 'CeDeROM' CEDRO <tomek@cedro.info>

docs/library/machine.UART.rst

@@ -87,10 +87,22 @@ Methods
        When no pins are given, then the default set of TX and RX pins is taken, and hardware
        flow control will be disabled. If *pins* is ``None``, no pin assignment will be made.
 
+   .. note::
+     It is possible to call ``init()`` multiple times on the same object in
+     order to reconfigure  UART on the fly. That allows using single UART
+     peripheral to serve different devices attached to different GPIO pins.
+     Only one device can be served at a time in that case.
+     Also do not call ``deinit()`` as it will prevent calling ``init()``
+     again.
+
 .. method:: UART.deinit()
 
    Turn off the UART bus.
 
+   .. note::
+     You will not be able to call ``init()`` on the object after ``deinit()``.
+     A new instance needs to be created in that case.
+
 .. method:: UART.any()
 
    Returns an integer counting the number of characters that can be read without