Added documentation explaining the difference between lumens and luxes

[NthTensor]

Objective

• Fix Improve lightight source light values documentation #9809

Solution

• Describe the difference between lumens and luxes, and why some lightsources use one and some the other.

[stepancheg]

Lux is used here instead of lumens because a directional light illuminates all surfaces more-or-less the same way (depending on the angle of incidence). Lumens can only be specified for light sources which emit light from a specific area.

Technically more correct explanation might be something like this:

... because for infinitely far directional light source, illuminance in lumens would be infinite which is practically useless.

[stepancheg]

more-or-less the same way (depending on the angle of incidence)

This can be rephrased like:

amount of light projected onto the surface depends only on the angle between the surface and the light direction, but not on the position in space.

[NthTensor]

Those would probably have been better. Do you feel they are important enough for me to open a follow-up?