Docs should be explicit about the comparator used in union, difference, and intersection

[dlee]

The uniq function explicitly states that the === operator is used to test for object equality. The union, difference, and intersection functions should also explicitly state how object equality is tested.

[jashkenas]

Actually, I think we could do with being a little less pedantic about value comparisons. All equality in Underscore is strict equality, unless the API says otherwise. In the case of uniq, the alternative is to pass an iterator function.