Document MIR dataflow framework

[ecstatic-morse]

rust-lang/rust#65672 just got merged, so we have a new framework for dataflow analyses in the compiler. I'm in the process of porting analyses that use BitDenotation to the new framework (see rust-lang/rust#68241). The new framework has decent in-line documentation, but I think newer contributors would benefit greatly from a high-level overview of dataflow analysis in rustc.