Add documentation for listing API

Add parameter to define for the sorting criteria of the returned blobs. @xla 
Instead of generic terms those should map to fields returned by the listing. If every blob entry has a field `key` that should be the value given to the sorting. Order could be expressed with `+` or `-`, e.g.:

`/{bucket}/?prefix={prefix}&sort=+key` - Would return sorted by the key field in ascending order.