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Several (doc) comments were super outdated or didn't provide enough context.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_hir_analysis/src/check/compare_impl_item.rs

@@ -746,7 +746,7 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
 
     // We may not collect all RPITITs that we see in the HIR for a trait signature
     // because an RPITIT was located within a missing item. Like if we have a sig
-    // returning `-> Missing<impl Sized>`, that gets converted to `-> [type error]`,
+    // returning `-> Missing<impl Sized>`, that gets converted to `-> {type error}`,
     // and when walking through the signature we end up never collecting the def id
     // of the `impl Sized`. Insert that here, so we don't ICE later.
     for assoc_item in tcx.associated_types_for_impl_traits_in_associated_fn(trait_m.def_id) {