I would recommend to add an explanation, what Query parameters are. So in the examples I would add:

router.get("/user") { request, context in
    // access the query parameter ?id=...
    let id = request.uri.queryParameters.get("id", as: Int.self) else { throw HTTPError(.badRequest) }
    return getUser(id: id)
}

and

...
router.get("tile") { request, context in
    // access the query parameters ?x=...&y=...
    let position = request.uri.decodeQuery(as: Coordinate.self, context: context)
    return tiles.get(at: position)
}

Also I would suggest to add the sentence:

If you don't need all parameters, mark the properties in the struct as Optional.

Maybe you also can add:

The parameters will automatically be percent encoded

@JoachimM7 I've added code comments about URL query format and a bit of text about percent decoding.

I'm not certain about the optional bit as this is kinda assumed knowledge when using Codable. I'm not generating general purpose swift documentation. Also the text isn't totally correct. In a way the text should be. If the existence of a query parameter is optional then they should be marked as Optional in the type you are decoding to. ie if is optional then mark it as optional.