Rollup merge of #144478 - joshtriplett:doc-code-formatting-prep, r=Amanieu

Improve formatting of doc code blocks

We don't currently apply automatic formatting to doc comment code blocks. As a
result, it has built up various idiosyncracies, which make such automatic
formatting difficult. Some of those idiosyncracies also make things harder for
human readers or other tools.

This PR makes a few improvements to doc code formatting, in the hopes of making
future automatic formatting easier, as well as in many cases providing net
readability improvements.

I would suggest reading each commit separately, as each commit contains one
class of changes.

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -425,7 +425,7 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for RemapLateParam<'tcx> {
 ///
 /// trait Foo {
 ///     fn bar() -> impl Deref<Target = impl Sized>;
-///              // ^- RPITIT #1        ^- RPITIT #2
+///     //          ^- RPITIT #1        ^- RPITIT #2
 /// }
 ///
 /// impl Foo for () {