Documentation of ThinkSubs notification scope and subscription sharing

The ThinkSubs documentation now covers three things that previously surprised people:

• Notifications are per account, by design of the application. There is no shared configuration; your channels, credentials and reminder timing affect only you.
• Email reminders are pre-filled for every account, with everything else — enabled state, sender address, extra recipients and every non-email channel — left entirely under your control.
• A subscription belongs to one account. Sharing a list between two logins is not possible in the application today (upstream #865); a mirror is available on request, and its one-way, never-deletes behaviour is spelled out.

Covered in the Getting Started guide, the FAQ and the architecture page.