docs(Pool): Add documentation for `Pool`

[bIgBV]

Add type level documentation for Pool. Update exports to include clear::Clear and clean them up.

[hawkw]

Also: we may want to update the lib.rs docs (and README) to note that this crate has a separate Pool type. But we can do that in a follow-up.

[bIgBV]

I was going to update the readme as well, but I realized that I would have to link to the Clear trait in it, so I'll create a separate PR for that once this gets merged in.