How to document repetetive parts of the grammar

[matthewjasper]

In #264 @brauliobz says

Also, there must be a ExpressionNoStructLiteral and a ExpressionStmtExpr that other productions use, but I'm still trying to find a good way to show this without cluttering the grammar.

As a first idea I would propose that the productions used for these (e.g. ComparisonExpressionNoStructLiteral) are only in the grammar appendix, once that exists, and we just link to them with a note explaining why.

[brauliobz]

In the reference we aren't constrained by parser generator limitations, so in my opinion we can use high level notation (not available in parser generators) or even prose in some of its productions if it doesn't lead to ambiguities.

[Havvy]

Bikeshed note, I'd like to see it be ExpressionExceptStructLiteral and then it'd just be pointing at Expression with the note saying that struct literals aren't allowed.