docs/library: Specify new machine.ADC class.

This initial specification is only for the ADC constructor and read_u16()
method.

docs/library/machine.ADC.rst

@@ -0,0 +1,35 @@
+.. currentmodule:: machine
+.. _machine.ADC:
+
+class ADC -- analog to digital conversion
+=========================================
+
+The ADC class provides an interface to analog-to-digital convertors, and
+represents a single endpoint that can sample a continuous voltage and
+convert it to a discretised value.
+
+Example usage::
+
+   import machine
+
+   adc = machine.ADC(pin)   # create an ADC object acting on a pin
+   val = adc.read_u16()     # read a raw analog value in the range 0-65535
+
+Constructors
+------------
+
+.. class:: ADC(id)
+
+   Access the ADC associated with a source identified by *id*.  This
+   *id* may be an integer (usually specifying a channel number), a
+   :ref:`Pin <machine.Pin>` object, or other value supported by the
+   underlying machine.
+
+Methods
+-------
+
+.. method:: ADC.read_u16()
+
+   Take an analog reading and return an integer in the range 0-65535.
+   The return value represents the raw reading taken by the ADC, scaled
+   such that the minimum value is 0 and the maximum value is 65535.