Fix some outdated help text about "Reply All" in "metamta.one-mail-per-recipient"

Summary: Ref T12240. When you "Reply All" to a Phabricator mail, we make an effort not to send the response to recipients who you hit with the original message. This isn't perfect and we can't always get it right, but the old description implies it's a bigger problem than it should be in practice.

Test Plan: Read text.

Reviewers: chad, eadler

Reviewed By: chad

Maniphest Tasks: T12240

Differential Revision: https://secure.phabricator.com/D17331