docs: add info on how to reset Export rangetest.csv

[edmkng]

On a Heltec, even after wiping and restalling the firmware, the rangetest.csv still lives on the device.

I.e. there's no way to reset the CSV file.

I haven't tested this on the Tbeam.

[andrekir]

if you're using the "Export rangetest.csv" feature, to reset the values go to the Debug Panel and press the "clear" button.

I'll make a note to add this to the docs.