Copter: add warning to AutoTune page that crashes may occur

We quite regularly see crashes from users attempting [AutoTune](https://ardupilot.org/copter/docs/autotune.html).  [Here's a recent example from 4.5.0-beta1 testing](https://discuss.ardupilot.org/t/copter-4-5-0-beta1-available-for-beta-testing/112504/11).

We should add a warning about ESC desync risks to the page and point them towards the Lua script.
