Add tip to disable improve location accuracy when using allow-mock-accuracy

[yanokwa]

In https://forum.getodk.org/t/gps-location-switching-between-tablet-location-and-mock-location/55466/8, @grzesiek2010 said....

Based on our testing, if you disable "Improve location accuracy" in the Android settings, the device will rely solely on GPS. In that case, if GPS is being spoofed, the location will consistently appear fake.

I think we should add this as a tip to the allow-mock-accuracy section in the docs. I'd put it where the selected-at() tip is currently and move that one before the XLSForm with optional parameters section.