Rollup merge of #153432 - nnethercote:dataflow-comments, r=cjgillot

Fix some comments about dataflow analysis.

Mostly in the examples in `initialized.rs`. In particular, the `EverInitializedPlaces` example currently doesn't cover how it's initialization sites that are tracked, rather than local variables (that's the `b_0`/`b_1` distinction in the example.)

r? @cjgillot

Author: JonathanBrouwer