R1:6e5df2dd714e

Document that disabling "metamta.one-mail-per-recipient" leaks recipients for "Must Encrypt"

Summary:
Depends on D19013. Ref T13053. When mail is marked "Must Encrypt", we normally do not include recipient information.

However, when `metamta.one-mail-per-recipient` is disabled, the recipient list will leak in the "To" and "Cc" headers. This interaction is probably not very surprising, but document it explicitly for completeness.

(Also use "Mail messages" instead of "Mails".)

Test Plan: Read…