docs: fix setup instructions for Mailgun and S3

Summary: The S3 fields are mandatory and if you only enter the ones in the docs you will immediately encounter a "Amazon S3 is Only Partially Configured" error. For Mailgun the error is more difficult to figure out - emails get stuck on Mailgun's side but the error is illegible. On Phabricator's side you have to go trawling through nginx logs to find "Mail signature is not valid. Check your Mailgun API key."

Test Plan: Deploy a new standalone instance, follow old instructions, fail. Deploy another one, follow updated instructions, win.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15931