Provide better general high-level documentation on mail commands

Summary: Ref T7199. This makes the page look less janky and provides more context about how mail commands work and how to use them.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12245