Provide some hints for Amazon RDS configuration

Summary: Fixes T2605. Provide some instructions on configuring RDS properly. The "DB Parameter Group" thing in the web UI seems pretty easy to use, it's just not obvious that it's what you should be using.

Test Plan: Jiggled these warnings to trigger them, viewed the output, saw a table of values and a hint about RDS.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2605

Differential Revision: https://secure.phabricator.com/D10343