More documentation

I've added documentation in the models so that it's easier for me and others to understand what each model and most methods do.

Fixed in next push (based on comments):

- 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`

Most notably, corrected errors in the description of `Interest` (`Subscription` now just points to `Interest`).