Rollup merge of #152587 - lqd:tiny-things, r=jackh726

A couple of tiny polonius things

Here's a couple of tiny things I had ready to go @jackh726
- a tiny cleanup to avoid round-tripping through `Location`s to check for liveness, since we're already working with points
- while I was there, I fixed the doc which wasn't showing up properly (maybe a rustdoc or bootstrap bug when we build locally or during dist, either way, both don't show up correctly linked most of the time) for `PointIndex`es.
- and in the second commit slightly expand test coverage with [an example](https://internals.rust-lang.org/t/get-mut-map-back-from-entry-api/24003) that would have needed stdlib changes (cc author @Darksonn for visibility here)

More soon.

r? @jackh726