Fix some outdated help text about "Reply All" in "metamta.one-mail-per-recipient"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

src/applications/config/option/PhabricatorMetaMTAConfigOptions.php

@@ -61,9 +61,10 @@ of each approach are:
       a normal recipient and also Cc'd on a mailing list.
     - Getting threading to work properly is harder, and probably requires
       making mail less useful by turning off options.
-    - Sometimes people will "Reply All" and everyone will get two mails,
-      one from the user and one from Phabricator turning their mail into
-      a comment.
+    - Sometimes people will "Reply All", which can send mail to too many
+      recipients. Phabricator will try not to send mail to users who already
+      received a similar message, but can not prevent all stray email arising
+      from "Reply All".
     - Not supported with a private reply-to address.
     - Mails are sent in the server default translation.
   - One mail to each user:
@@ -73,7 +74,8 @@ of each approach are:
       mail.
     - Getting threading to work properly is easier, and threading settings
       can be customzied by each user.
-    - "Reply All" no longer spams all other users.
+    - "Reply All" will never send extra mail to other users involved in the
+      thread.
     - Required if private reply-to addresses are configured.
     - Mails are sent in the language of user preference.