Clarify subscription terminology

[novoselt]

Some users are rightfully confused by getting emails "your subscription will renew" when they have no subscriptions active. The problem is that we have "CoCalc subscriptions" attached to licenses and "Stripe subscriptions" (called "CoCalc Automatic Billing") attached to the whole account when user enables "Automatic payments".

There has to be more clarity about it

• on the site
• in the documentation
• in the email from Stripe if we can control it

For the site and documentation part, it would help to address #7091 first.

[williamstein]

Please make this more precise.