Add docstrings, mention that DB drivers are required for binaries

[vindarel]

Hi, these are two simple QoL improvements: some docstrings, stating the obvious but I find helpful for exploring on the REPL, and a more useful example.

thanks

[vindarel]

related: https://github.com/fukamachi/cl-dbi/pull/89 (but 99% of users will stick to Mito's documentation)

cl-dbi installs the DB driver on the fly if it doesn't find it, but this can easily fail with a binary on a server.

[fukamachi]

Thank you!