Clarify documentation for `Servant.Links.allLinks`

The haddocks for [`allLinks`](https://hackage.haskell.org/package/servant-0.20.1/docs/Servant-Links.html#v:allLinks) have the following note:

> nested APIs don't work well with this approach

and an example is given.

Does that mean that for nested APIs the links aren't correctly generated, or simply that the resulting "link builder" values like `Char -> (Int -> Link) :<|> (Double -> Link)` are unwieldly to use?

In the second case, perhaps it should be mentioned that `allLinks` still works when the nested APIs are represented with route records (video example [here](https://youtu.be/KC64Ymo63hQ?t=146)) and the ergonomics are better with records. Perhaps a doctest example could be given.

Start with the `allLinks` Haddock in `Servant-Links` and inspect the nested API example described in the issue. Clarify whether nested links are incorrect or merely awkward, and document the behavior, including route-record ergonomics or a doctest example if supported by the existing documentation.

Written by the indexing model from the issue text.