docs/esp32: Mention Signal in GPIO section of quickref.

docs/esp32/quickref.rst

@@ -171,6 +171,10 @@ Notes:
 * The pull value of some pins can be set to ``Pin.PULL_HOLD`` to reduce power
   consumption during deepsleep.
 
+There's a higher-level abstraction :ref:`machine.Signal <machine.Signal>`
+which can be used to invert a pin. Useful for illuminating active-low LEDs
+using ``on()`` or ``value(1)``.
+
 UART (serial bus)
 -----------------