Fix misleading explanation of comma in $(),*

[pitaj]

The previous explanation makes it sound like the comma in that place would match an optional trailing comma like in [a, b, c,]. That is incorrect, and actually that only matches commas between elements.

Motivation: https://stackoverflow.com/q/77701292/847382

[chriskrycho]

This is reasonably well-motivated; I see why it confused the Stack Overflow user. I want to think a little about phrasing here—your suggestion is good, but this is a little tricky and I want to make sure we get it just right!