Add documentation for new connection pool settings and remove some

`:retry_timeout` and `:retry` got removed because reconnecting on failure is
something the new pool intentionally doesn't do.

`:max_overflow` had to go in favor of `:max_waiting`, I didn't reuse the key because
the settings are very different in their behaviour.

`:checkin_timeout` got removed in favor of `:connection_acquisition_wait`,
I didn't reuse the key because the settings are somewhat different.

I didn't do any migrations/deprecation warnings/changelog entries because
these settings were never in stable.