docs/esp8266: Add quickref documentation for UART on esp8266.

This patch adds quickref documentation for the change in commit
afd0701bf7.  This commit added the ability to
disable the REPL and hence use UART0 for serial communication on the
esp8266, but was not previously documented anywhere.

The text is largely taken from the commit message, with generic information
on using the UART duplicated from the Wipy quickref document.

docs/esp8266/quickref.rst

@@ -138,6 +138,45 @@ Also note that Pin(16) is a special pin (used for wakeup from deepsleep
 mode) and may be not available for use with higher-level classes like
 ``Neopixel``.
 
+UART (serial bus)
+-----------------
+
+See :ref:`machine.UART <machine.UART>`. ::
+
+    from machine import UART
+    uart = UART(0, baudrate=9600)
+    uart.write('hello')
+    uart.read(5) # read up to 5 bytes
+
+Two UARTs are available. UART0 is on Pins 1 (TX) and 3 (RX). UART0 is
+bidirectional, and by default is used for the REPL. UART1 is on Pins 2
+(TX) and 8 (RX) however Pin 8 is used to connect the flash chip, so
+UART1 is TX only.
+
+When UART0 is attached to the REPL, all incoming chars on UART(0) go
+straight to stdin so uart.read() will always return None.  Use
+sys.stdin.read() if it's needed to read characters from the UART(0)
+while it's also used for the REPL (or detach, read, then reattach).
+When detached the UART(0) can be used for other purposes.
+
+If there are no objects in any of the dupterm slots when the REPL is
+started (on hard or soft reset) then UART(0) is automatically attached.
+Without this, the only way to recover a board without a REPL would be to
+completely erase and reflash (which would install the default boot.py which
+attaches the REPL).
+
+To detach the REPL from UART0, use::
+
+    import uos
+    uos.dupterm(None, 1)
+
+The REPL is attached by default. If you have detached it, to reattach
+it use::
+
+    import uos, machine
+    uart = machine.UART(0, 115200)
+    uos.dupterm(uart, 1)
+
 PWM (pulse width modulation)
 ----------------------------