Clarify what can/should be close! ed

[davidanthoff]

Right now the README looks like one can close statements, but presumably one can also close results and connections?

Maybe it would be good if the convention for users of the API is that they actually should call close! on connections, statements and results?

[quinnj]

Ok, we've recently updated the docs on close! and I just fixed the docs generation, which can be seen here: https://juliadatabases.org/DBInterface.jl/dev/