Document that 1 unit = 1 meter in Bevy's PBR rendering

## How can Bevy's documentation be improved?

I was porting some code from a different engine and spent quite some time looking why point lights were not working.
The main issue was that my meshes had sizes of up to 2000 and point lights would never illuminate those even with a high range. Also when I got those lights to work somewhat they would turn off when I zoomed out.

I did a search for units or metrics but only somewhere on discord did I find that I had to scale everything to meters as lights are based on real-world units.

Maybe I missed some existing documentation on this?