Document default values (1100-1900) for RC Inputs and Outputs

Currently we don't specify default values for [RC Inputs and Outputs](../blob/master/developers/rc-input-and-output.md). Would be good to indicate that they're 1100-1900 (instead of the 1000-2000 used by other ArduPilot firmwares), and ideally explain why as well (not sure why yet - have asked internally).