rustdoc: Fix many rustdoc errors (#31147)

This fixes many rustdoc errors that occur due to raw URLs in rustdoc
comments as well as unescaped Rust code that should be in backticks.

components/layout/display_list/background.rs

@@ -40,7 +40,7 @@ pub struct BackgroundPlacement {
 /// Obviously it does not work with empty arrays.
 ///
 /// This is used for multiple layered background images.
-/// See: https://drafts.csswg.org/css-backgrounds-3/#layering
+/// See: <https://drafts.csswg.org/css-backgrounds-3/#layering>
 pub fn get_cyclic<T>(arr: &[T], index: usize) -> &T {
     &arr[index % arr.len()]
 }

components/layout/display_list/webrender_helpers.rs

@@ -103,7 +103,7 @@ impl<'a> ClipScrollState<'a> {
 }
 
 /// Contentful paint, for the purpose of
-/// https://w3c.github.io/paint-timing/#first-contentful-paint
+/// <https://w3c.github.io/paint-timing/#first-contentful-paint>
 /// (i.e. the display list contains items of type text,
 /// image, non-white canvas or SVG). Used by metrics.
 pub struct IsContentful(pub bool);