Add short mentions about "generic" fallback for "regular" and digest" footer and header template options in Postorius template settings

This is a child of mailman#1243. Please check it off there when closing this issue.

The template settings and docstrings are stored in the Postorius database, so to update this documentation, you need to update the database, then create a Django database migration.

See post for suggested wording and the containing threada for context.