Various documentation tweaks based on feedback:

- fix docstring for Interest#in
- remove XXX near Interest#create_subscriptions
- add more docs to SeenItem class
- add more descriptive @todo for SeenItem#subscription_id
- remove SeenItem#json_view
- add more descriptive @todo for Subscription#query
- replace docstring with @see for Subscription#search
- made some # internal comments into # @private