Add leds docs; apply cargo fmt

[brownjohnf]

I've added more explicit documentation around how to interact with the
leds module. I also ran cargo fmt against the repo, which found some
changes.

[rubberduck203]

Thanks!

Unfortunately, it's hard to review this with the formatting changes mixed in.
I was intentionally not using cargo fmt on this repo because it did some weird things.
Maybe those things are fixed now and I'd consider it going forward,
but please separate the formatting into a different pull request.