Should remove or formalize `Base.delete`

Julia v1.11 docs for [PersistentDict](https://docs.julialang.org/en/v1.12-dev/base/collections/#Base.PersistentDict) use `Base.delete` in the examples but it's not part of the public API. It's a bit misleading to use internal/private methods in public API docs. This should probably be formalized or removed from the doc string.