Better clarify announcements notification setting

Finishing https://gitlab.com/gitlab-org/gitter/webapp/-/merge_requests/1785

Fixes https://gitlab.com/gitlab-org/gitter/webapp/issues/2456

Thanks @ianfixes for clarifying the notification settings till #1205 gets fixed 🎉

I paired the You will get emails for all unread messages which we know might be undesirable, see #1205 message with the Show unread item counts message because that't how it is implemented.