Figure out long term role of `ValueHint`

[epage]

ValueHint works well for ahead-of-time generated completions as it tells the generator what shell behavior to opt-in to.

However, its not really needed with Rust-native completions and actually puts more of a support burden on it.

On top of that, the documentation default is less relevant now that we have typed values. We changed it for Rust-native completions in #5763 as that is still unstable.