Annotate all methods with Arg types and Return types

[zcmarine]

As other devs being to add features to pgbedrock, it would be helpful to have more detailed docstrings, similar to datasheets: show the arg types and their return types. If we wanted, we could even add type checking, though I think (?) we'd have to drop Python 2 support to do that. Still, it might be worth it.