described matrix channelOrder in more detail

[moritzvieli]

Based on the documentation, it was not clear to me, what the channelOrder does exactly. I found some more information here and tried to describe it a little bit more in detail in the docs: https://github.com/OpenLightingProject/open-fixture-library/blob/715e90ca0af667c4e936f853afec6d5befaad2e6/tests/fixture-valid.js#L789

However, I'm not sure, whether it should be the other way round. Based on the code, it should look like this, but based on the naming, I think it should be the other way round.

I found no fixture already using perChannel currently.