Add more explanation of known vs subscribed lists to manual.

Add more explanation of known vs subscribed lists to manual.

Mention in both the "Mailing Lists" and "Handling Mailing Lists"
sections the difference between known and subscribed mailing lists.

Note that the Mail-Followup-To header is generated if any of the
recipients are known mailing lists.  If any of those are subscribed
mailing lists, the sender's address will not be in the header.  If all
of them are known (but not subscribed), the sender's address will be
added to the header.