Updated ADC API docs

Author: dplanella

docs/ADC.rst

@@ -13,7 +13,7 @@ Example::
     # The read method returns normalized values from 0 to 1.0
     value = ADC.read("P9_40")
 
-    # The read_raw returns non-normalized values
+    # The read_raw returns non-normalized values from 0 to 4095
     value = ADC.read_raw("P9_40")
 
 .. module:: Adafruit_BBIO.ADC
@@ -24,13 +24,22 @@ Example::
 
 .. function:: setup_read(channel)
 
+   Read the normalized analog value for the channel.
+
    :param str channel: GPIO channel to read the value from (e.g. "P8_16").
+   :returns: normalized value reading from 0.0 to 1.0
+   :rtype: float
 
-   Read the normalized 0-1.0 analog value for the channel.
 
 .. function:: setup_read_raw(channel)
 
+   Read the raw analog value for the channel.
+   
+   :note: Kernels older than 4.1.x returned a raw value range based on
+       the reference voltage of 1.8 V–– from 0 to 1800.
+
    :param str channel: GPIO channel to read the value from (e.g. "P8_16").
+   :returns: raw value reading from 0 to 4095 (12 bits).
+   :rtype: float
 
-   Read the raw analog value for the channel.