[Feature] Document unit convention (pixels v microns) clearly

[alessandrofelder]

Is your feature request related to a problem? Please describe.
https://forum.image.sc/t/brainmapper-points-transformation-issue/105261/6

Describe the solution you'd like
In particular, it's confusing that brainrender and brainrender-napari have differing conventions (because we want tools within napari to be self-consistent) - so this should be very clearly documented. And the fact that our "Export to brainrender" button does the scaling for you.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[imagesc-bot]

This issue has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/brainmapper-points-transformation-issue/105261/7