Clarify what Unique iterator means

[dennwc]

Currently Unique iterator only affects node IDs. But for real queries it's usually necessary to extract unique records (sets of tags). The most notable example is SavePredicates - user might expect results to be unique, but since it only affects nodes, it will return only one predicate from each connected node, or will duplicate predicate multiple times if more than one node can be traversed to via the same predicate.