R1:cd7ba6cd7bf9

Improve documentation around using Mail stamps with Gmail

Summary: Ref T13069. See PHI54. Some of this behavior isn't entirely obvious, so give users a heads up in the documentation to help warn them about what is to come.

Test Plan: Read documentation.

Maniphest Tasks: T13069

Differential Revision: https://secure.phabricator.com/D19227