(#2852) - explain data formats

As I continue fixing attachment stuff, it occurs to me that it may be helpful to talk about PouchDB's data format.

One thing that sets us apart from other IndexedDB/WebSQL shims is that we are actually pretty damned efficient about storing binary data. We should inform users about that.

Also, the issue of "can I store non-JSON" has come up a lot, so I think it's worth documenting in the FAQs.