Change internal voltage unit in docs

Now we're using millivolts internally rather than centivolts.  This
might be a good choice if USB-IF decides to make even more finely
adjustable voltages in the future.  If they don't, well, it still allows
voltages up to 65.535 V, which is beyond what some electrical codes
consider "low voltage", so I doubt USB will ever exceed that.

docs/console_config.md

 # PD Buddy Sink Serial Console Configuration Interface
 
-Version 1.2.0-dev, 2017-10-10
+Version 1.2.0-dev, 2017-12-09
 
 The PD Buddy Sink can be put into setup mode by holding the Setup button while
 plugging it into a computer.  In this mode, the device runs a configuration
 Sets the voltage of the configuration buffer, in millivolts.  Prints no output
 on success, an error message on failure.
 
-Note: values are rounded down to the nearest 10 mV.
+Note: values are rounded down to the nearest 20 mV, 50 mV, or 100 mV for
+various parts of the USB Power Delivery protocol.
 
 #### set_vrange
 
 Sets the minimum and maximum voltage of the configuration buffer, in
 millivolts.  Prints no message on success, an error message on failure.
 
-Note: values are rounded down to the nearest 10 mV.
+Note: values are rounded down to the nearest 20 mV, 50 mV, or 100 mV for
+various parts of the USB Power Delivery protocol.
 
 #### set_i