Make `Queryable` implementations explicit

Currently in docs we see:
```
impl Queryable for T
```
Which doesn't give a list of scalar types that implement that trait.

Note: I don't think that alternative of exposing `DecodeScalar` is a good idea.

No files, tests, or entry points are named in the issue. Start by locating the Queryable and DecodeScalar implementations, then inspect how their generated documentation is produced; done means the documentation lists the scalar types explicitly without exposing DecodeScalar.

Written by the indexing model from the issue text.