[docs] Parens create a pluck context

[hildjj]

It's not clear from the docs that this is legal:

foo = "a" b:(@"c" "d") { return 2 }

because the parens create a new context for returns. Same with the old way:

foo = "a" b:(first:"c" "d" { return first })

[hildjj]

Parens also create a location() context.

[hildjj]

I figured I'd fix 398 while I was in there.

[reverofevil]

Oh, BTW, do you remember when it was introduced? I remember at least old versions of PEG.js didn't have this kind of scoping.

[hildjj]

Probably in 2.0. It's been in for a while, but definitely after we forked.

[hildjj]

It was 1.1:

peggy/CHANGELOG.md

Line 371 in dfca04c

- Back-ported value plucking with `@` from pegjs head. If your rule has a simple action that returns one or more of the values matched by the rule, you can instead mark those expressions with `@` and not need an action. This works inside of parens as well. [@hildjj](https://github.com/peggyjs/peggy/pull/89)

[reverofevil]

Thanks!

It was one of the larger improvements to DX, but I didn't know it happened after the fork.

[hildjj]

It was technically after the fork, because we forked from the last-known-good spot, then the plucks were back-ported from head -- because that was the feature I wanted to be released enough to want the fork. :)