Add analogReadResolution() docs (#631)

Author: mnltake

docs/analog.rst

@@ -13,6 +13,11 @@ int analogRead(pin_size_t pin = A0..A3)
 Returns a value from 0...4095 correspionding to the ADC reading
 of the specific pin.
 
+void analogReadResolution(int bits)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Determines the resolution (in bits) of the value returned by the analogRead() function.
+Default resolution is 10bit.
+
 float analogReadTemp()
 ~~~~~~~~~~~~~~~~~~~~~~
 Returns the temperature, in Celsius, of the onboard thermal sensor.