Add a note about the functioning of webc:setup

[harrislapiroff]

I believe this note is accurate to how webc:setup works, based on this message from a Discord user:

webc:setup only runs once for the whole project - it doesn't have access to slots or anything else specific to some instance of the component. (But you can define functions there, then pass them the local data. I'd guess that slots could be passed in too, but I'm not sure if that's only available in webc:type="js".)

If this is indeed a correct understanding, I think this note would be helpful to new WebC coders in understanding the limitations and best practices with webc:setup