feat: generalize select_n dispatch

[nstarman]

Now the dispatch uses a non-Q selector for a set of Q cases.

Addresses problem identified in private communication.

[nstarman]

The example in the docs will get run as a doctest, right? So don't need to add a separate unit test?

Yes!

[nstarman]

@adrn do you need a bugfix release?

[adrn]

Not urgently