Update (doc) comments

Several (doc) comments were super outdated or didn't provide enough context.

Some doc comments shoved everything in a single paragraph without respecting
the fact that the first paragraph should be a single sentence because rustdoc
treats these as item descriptions / synopses on module pages.

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -1264,7 +1264,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
                 // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`,
                 // normalizing in the process, since `type_of` returns something directly from
-                // astconv (which means it's un-normalized).
+                // HIR ty lowering (which means it's un-normalized).
                 let source_tail = normalize_with_depth_to(
                     self,
                     obligation.param_env,