Remove 'automatic subscriptions' key phrase

[rossjrw]

I don't like 'automatic subscriptions' as a key phrase that generically covers all the ways a user might be automatically subscribed to something. I don't think it even makes sense; subscription implies an action taken to get to that point, it should be synonymous with 'manual subscription', but here we are.

All places that mention 'automatic subscription' should be replaced with a list of what that actually means. It's like 2 things right now, more soon, and it'd be good to be as explicit as possible at all times.

Requires:

• Changes to documentation
• Changes to lexicon