Briefly document mail stamps and remove obsolete header documentation

Summary: Fixes T10189. Ref T13053. We haven't sent these headers in a very long time. Briefly mention the new stamps header instead, although I expect to integrate stamp documentation into the UI in a more cohesive way in the future.

Test Plan: Read documentation.

Maniphest Tasks: T13053, T10189

Differential Revision: https://secure.phabricator.com/D19030