docs/esp32: Mention the use of Timer(0) by UART.IRQ_RXIDLE.

In both the machine.UART and esp32.quickref sections.

Signed-off-by: robert-hh <[email protected]>

Author: robert-hh

docs/esp32/quickref.rst

@@ -284,7 +284,8 @@ with a timer ID of 0, 0 and 1, or from 0 to 3 (inclusive)::
     tim1 = Timer(1)
     tim1.init(period=2000, mode=Timer.PERIODIC, callback=lambda t:print(1))
 
-The period is in milliseconds.
+The period is in milliseconds. When using UART.IRQ_RXIDLE, timer 0 is needed for
+the IRQ_RXIDLE mechanism and must not be used otherwise.
 
 Virtual timers are not currently supported on this port.
 

docs/library/machine.UART.rst

@@ -224,7 +224,8 @@ Methods
 
 
    .. note::
-     - The ESP32 port does not support the option hard=True.
+     - The ESP32 port does not support the option hard=True. It uses Timer(0)
+       for UART.IRQ_RXIDLE, so this timer cannot be used for other means.
 
      - The rp2 port's UART.IRQ_TXIDLE is only triggered when the message
        is longer than 5 characters and the trigger happens when still 5 characters