AP_Periph: clarify RNGFNDx_MIN/MAX parameters need to be set within the Periph device and the AP flight code

[rmackay9]

Somewhere on the wiki (I'm not quite sure where) we specify how to setup the AP_Periph rangefinders and specify that the RNGFNDx_ORIENT should be "0" on the AP_Periph side. This is fine but we should also clarify that the RNGFNDx_MIN_CM and RNGFNDx_MAX_CM need to be set on both the AP_Periph side and within within the Autopilot flight code parameters.

The parameters should be consistent as well and while it's sad users need to set the MIN/MAX in both places, it is necessary because they're used for different things.

• the AP_Periph side MIN/MAX parameters are used to set the state of the rangefinder (e.g. range too low, range too high, range OK). The AP_Flight code side consumes the state from the dronecan rangefinder message
• the AP Flight code MIN/MAX parameters are used for various other purposes. One example is within the arming checks. If a user sets RTL_ALT_TYPE = 1 (Terrain) we check that RNGFNDx_MAX_CM (within the flight code parameters) is less than RTL_ALT.

[Hwurzburg]

"Somewhere on the wiki (I'm not quite sure where) we specify how to setup the AP_Periph rangefinders " not that I can find...perhaps a note in the https://github.com/ArduPilot/ardupilot/blob/master/Tools/AP_Periph/README.md? for anything a bit strange like this rangefinder stuff?we dont have a lot of detail in the dev section other than params and general stuff....more info could be added but just a note on thos would be out of place and context without more extensive info on AP peripherals internal param setups for other devices also