Update dash.el

Hello,

I notice some big performance differences in calls of `-union` on my (rather slow) computer: computing the union of an empty list and a list with ~5000 elements takes about 1.5s, while computing the union of the same ~5000-long list and an empty list takes 0.02s.

The reason for that is clear by the implementation, and I don't think it should be changed. But I suggest a change in the docstring to make users aware of this phenomenon.

Note that other functions may show the same behavior, I only use a small subset of the library so I wouldn't know.