Update doc for rand(range) and replace Range1 -> UnitRange

I'm not certain that this new doc for rand(range) is clear enough.
Your are right, and I admit that I only did a search & replace here. However, as you pointed out at https://github.com/JuliaLang/julia/issues/7782#issuecomment-50512760, ranges are not even documented so a new pass will be necessary when they are. And I just notice now that `DNRange` exists nowhere except in the above line (I can't fix as I don't know by what this should be replaced).