Clarify the expected conduct for debian-private mails

Some persons (such as mail admins) might get confused by
the request to not forward mail whilst their @debian.org
address automatically does that. Use "other people" instead
of "elsewhere" to clarify that.

Switch from "mail" to "info" since folks could also forward info
over other transports such as instant messengers.

Split the #debian-private IRC channel info to a separate paragraph.