Added documentation explaining the difference between lumens and luxes #35359

Job	Run time
build (windows-latest)	4m 24s
ci	2m 7s
build (ubuntu-latest)	2m 29s
check-bevy-internal-imports	7s
build (macos-latest)	4m 32s
check-doc	6m 8s
check-missing-examples-in-docs	35s
miri	6m 34s
run-examples-on-windows-dx12	14m 13s
toml	4s
check-missing-features-in-docs	28s
markdownlint	1m 9s
check-compiles	3m 27s
build-wasm	47s
msrv	1m 10s
	48m 14s

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