Improve mango docs

[aenario]

Following PO testing of [DS-13 / E-20] , we identified some issues in docs.

• Using {} selector to get all docs was a hack and broke when we forbid unoptimised queries, remove it from the docs. We should replace it by an HTTP API for couchdb.GetAllDocs (see [APPS] Improve error handling #100)
• Sort field must be an array and match an existing index,.

And @jinroh saw that it is more efficient to use_index, in couchdb docs

For now, the API is just a proxy to couchdb, so we only improve docs to match API behaviour.
Later we might imagine improving API to match user expectation, like autocompleting the beginning of sort field.