Update param types for addRegion method

@thejameskyle LMK if this is correct
Oooh. I just realized that the multi-line params are for *multiple* parameters. Okay, I think it should just be what you originally suggested, @jdaudier, where you just specify `Function`, then elaborate on it in the description.

Constructors *are* just functions, so we're not even making any compromises here. We're tellin' it exactly like it is.