For children of syntax nodes that are TokenSyntax, list the kinds of
tokens it can contain. Fixes swiftlang#1987.

- Moves complexity of generating the comment into `Child.swift` and uses
  `grammar.grammar()` directly.
- Appends a `TriviaPiece.newlines(1)` between trivia pieces when merging
  with the original comment.
- Uses approach similar to `SyntaxNodesFile.swift` to concatenating
  documentation pieces.

- Improves readability by omitting single quotes inside backticks
  (@kimdv)
- Cleans up the language on possible token choices (@ahoppen)
- Moves token choices generation into `GrammarGenerator`, but leaves the
  documentation property in `Child` so it's consistent with how `Node`
  works.
- Regenerated sources with the changes after this cleanup, and after
  rebaising on main.

We've introduced `Trivia(joining: [Trivia], separator: [Trivia])` to
generate doc comments from multiple pieces, and already use this
approach in `SyntaxNodesFile` for nodes themselves, and for node
children documentation.

This commit switches from joining pieces manually to using
`Trivia(joining:)` in `SyntaxCollectionsFile` so that we use the same
approach everywhere.