Add a rustdoc test for union to exhibit how it is used.

There is already a test for `union` in the test namespace, but this commit adds a doctest that will appear in the rustdocs.

Someone on IRC said, *Write doctests!*, so here I am.

I am not sure this is the best way to demonstrate the behavior of the union function, so I am open to suggestions for improving this. If I am on the right track I'd be glad to include similar doctests for `intersection`, `difference`, etc.
@Gankro I can do that. I was actually just noticing that that is more or less what is done for corresponding methods of some of the other data structures.