Added comment to clarify IMU acceleration value

[Tuxliri]

Clarifying IMU reading value, as noted in #29072.

[rmackay9]

Hi @Tuxliri,

Txs for this. Could you update the commit title to prefix it with "SITL: "? We like to prefix all our commits with the subsystem or folder name where the change is made because it makes backporting easier. To be clear, I meant the commit title, not the PR title.

[Tuxliri]

@rmackay9 done!

[IamPete1]

Did you find the readme? https://github.com/ArduPilot/ardupilot/blob/master/libraries/SITL/examples/JSON/readme.md Do you think we should add the clarification there too?

[Tuxliri]

@IamPete1 this is example-specific so not related to the JSON protocol per-se. But it would be useful to add a section to the README with a description of the reference frames used and this could be an example there. I have written a Jupyter notebook to explain this (link), I can sum it up in the readme.