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_2020/display_list/gradient.rs

@@ -55,7 +55,7 @@ pub(super) fn build(
     }
 }
 
-/// https://drafts.csswg.org/css-images-3/#linear-gradients
+/// <https://drafts.csswg.org/css-images-3/#linear-gradients>
 pub(super) fn build_linear(
     style: &ComputedValues,
     items: &[GradientItem<Color, LengthPercentage>],
@@ -155,7 +155,7 @@ pub(super) fn build_linear(
     )
 }
 
-/// https://drafts.csswg.org/css-images-3/#radial-gradients
+/// <https://drafts.csswg.org/css-images-3/#radial-gradients>
 pub(super) fn build_radial(
     style: &ComputedValues,
     items: &[GradientItem<Color, LengthPercentage>],
@@ -255,7 +255,7 @@ pub(super) fn build_radial(
     )
 }
 
-/// https://drafts.csswg.org/css-images-4/#color-stop-fixup
+/// <https://drafts.csswg.org/css-images-4/#color-stop-fixup>
 fn fixup_stops(
     style: &ComputedValues,
     items: &[GradientItem<Color, LengthPercentage>],

components/layout_2020/display_list/mod.rs

@@ -118,7 +118,7 @@ pub(crate) struct DisplayListBuilder<'a> {
 
     /// Contentful paint i.e. whether the display list contains items of type
     /// text, image, non-white canvas or SVG). Used by metrics.
-    /// See https://w3c.github.io/paint-timing/#first-contentful-paint.
+    /// See <https://w3c.github.io/paint-timing/#first-contentful-paint>.
     is_contentful: bool,
 }
 

components/layout_2020/display_list/stacking_context.rs

@@ -511,7 +511,7 @@ impl StackingContext {
         true
     }
 
-    /// https://drafts.csswg.org/css-backgrounds/#special-backgrounds
+    /// <https://drafts.csswg.org/css-backgrounds/#special-backgrounds>
     ///
     /// This is only called for the root `StackingContext`
     pub(crate) fn build_canvas_background_display_list(