Unrolled build for rust-lang#128085

Rollup merge of rust-lang#128085 - Zalathar:notes, r=Nadrieril

Various notes on match lowering

This is an assortment of comments for things that I found unclear or confusing when I was learning how match lowering works.

This PR only adds/modifies comments, so there are no functional changes.

I have tried to avoid touching code that would conflict with rust-lang#127159.

r? `@Nadrieril`

Author: rust-timer