Document database connections setup

[cirocosta]

I'm not 100% on the necessity of this in light of #244, but we've seen on the field people using https://pgbouncer.github.io/ in front of web nodes in order to better pool the connections that web nodes create against the db.

I wonder if we documented the behavior of lib/pq regarding idle connections we could better inform people of what to expect when it comes to database connections.

Wdyt?

Thanks!

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Related: concourse/concourse#4232