Fully document all keys returned by the API for an app (bug 932365)

https://bugzilla.mozilla.org/show_bug.cgi?id=932365

This should fully document the apps API, report any typos, broken or inconsistent english, etc.

There are a couple FIXMEs for payment stuff that I'd like some help with (I can verify myself but it'll be faster if someone from the payments team fills in the blanks)