Document grammar macro shorthands

[ReubenJ]

Make sure we explicitly document the 3 cases:

• |(1:3)
• |([1,2,3])
• 1|2|3

To discuss: possibly formalize the shorthand with a grammar of its own and pass off parsing to something like ParserCombinator.jl? That way we don't rely as much on reasoning about our implementation being correct.