Document pass-by-reference behavior for large arguments

[Feoramund]

This was discussed in odin-lang/Odin#2971 but I couldn't find any mention of it on the overview. I used @jakubtomsu's initial example with expanded names so the problem is more clear.

It would be good if we could get an exact and explicit definition of what is a "large" argument.

[jakubtomsu]

Good idea.
Btw I'm not sure if the calling convention uses different thresholds on different compilation targets though. It might be a good idea to standardize it to always be size_of(rawptr) * 2, (which is what linux RISC-V calling convention does for example)