Fix commonmark Markdown warnings in docs, part 1

Fixes warnings from rust-lang/rust#44229 when `--enable-commonmark` is
passed to rustdoc.

This is mostly a global find-and-replace for bare URIs on lines by
themselves in doc comments.

components/layout/display_list_builder.rs

@@ -858,7 +858,7 @@ fn get_ellipse_radius<F>(size: &Size2D<Au>, center: &Point2D<Au>, cmp: F) -> Siz
 }
 
 /// Determines the radius of a circle if it was not explictly provided.
-/// https://drafts.csswg.org/css-images-3/#typedef-size
+/// <https://drafts.csswg.org/css-images-3/#typedef-size>
 fn convert_circle_size_keyword(keyword: ShapeExtent,
                                size: &Size2D<Au>,
                                center: &Point2D<Au>) -> Size2D<Au> {
@@ -882,7 +882,7 @@ fn convert_circle_size_keyword(keyword: ShapeExtent,
 }
 
 /// Determines the radius of an ellipse if it was not explictly provided.
-/// https://drafts.csswg.org/css-images-3/#typedef-size
+/// <https://drafts.csswg.org/css-images-3/#typedef-size>
 fn convert_ellipse_size_keyword(keyword: ShapeExtent,
                                 size: &Size2D<Au>,
                                 center: &Point2D<Au>) -> Size2D<Au> {

components/layout/flex.rs

@@ -264,7 +264,7 @@ impl FlexLine {
     /// This method implements the flexible lengths resolving algorithm.
     /// The 'collapse' parameter is used to indicate whether items with 'visibility: collapse'
     /// is included in length resolving. The result main size is stored in 'item.main_size'.
-    /// https://drafts.csswg.org/css-flexbox/#resolve-flexible-lengths
+    /// <https://drafts.csswg.org/css-flexbox/#resolve-flexible-lengths>
     pub fn flex_resolve(&mut self, items: &mut [FlexItem], collapse: bool) {
         let mut total_grow = 0.0;
         let mut total_shrink = 0.0;

components/layout/query.rs

@@ -679,7 +679,7 @@ pub fn process_node_scroll_area_request< N: LayoutNode>(requested_node: N, layou
 }
 
 /// Return the resolved value of property for a given (pseudo)element.
-/// https://drafts.csswg.org/cssom/#resolved-value
+/// <https://drafts.csswg.org/cssom/#resolved-value>
 pub fn process_resolved_style_request<'a, N>(context: &LayoutContext,
                                              node: N,
                                              pseudo: &Option<PseudoElement>,