Document how to regenerate the config section of `package.json`

[lnicola]

Side note: Since docs/dev is lacking in that regard and it took me a while to figure out, you might want to mention somewhere that you only need to change config.rs and running cargo test will generate the rest.

Originally posted by @sclu1034 in #9051 (comment)

[matklad]

No comments in JSON strikes again!

[lnicola]

They said there's no need for comments in JSON because you can make a map: { "_": "hello there", ... } every time you want to put down a comment.