Document the D-term pre-diff LPF flight-test procedure

A repeatable flight-test procedure would make feedback on dterm_lpf2_hz easier to compare.

Useful sections:
- baseline with dterm_lpf2_hz disabled
- test values such as 200Hz and 250Hz
- conservative flight pattern
- Blackbox logging requirements
- motor temperature notes
- what to report back in the issue

Related PR: https://github.com/iNavFlight/inav/pull/11464