Rollup merge of #97249 - GuillaumeGomez:details-summary-fixes, r=notriddle

`<details>`/`<summary>` UI fixes

With images it's easier to understand:

![Screenshot from 2022-05-21 14-10-42](https://user-images.githubusercontent.com/3050060/169653038-9c58de67-589a-4986-a8ff-dbdddaf136a4.png)
![Screenshot from 2022-05-21 14-08-49](https://user-images.githubusercontent.com/3050060/169653042-56e87258-13fe-4f80-9858-4e15c318c3fb.png)

The headings in `<summary>` should not have bottom border so I removed it as well alongside the other fixes.

r? `@jsha`

src/librustdoc/html/static/css/rustdoc.css

@@ -158,8 +158,8 @@ h1.fqn {
 	Underlines elsewhere in the documentation break up visual flow and tend to invert
 	section hierarchies. */
 h2,
-.top-doc h3,
-.top-doc h4 {
+.top-doc .docblock > h3,
+.top-doc .docblock > h4 {
 	border-bottom: 1px solid;
 }
 h3.code-header {
@@ -1681,6 +1681,11 @@ details.rustdoc-toggle[open] > summary.hideme::after {
 	content: "Collapse";
 }
 
+/* This is needed in docblocks to have the "▶" element to be on the same line. */
+.docblock summary > * {
+	display: inline-block;
+}
+
 /* Media Queries */
 
 @media (min-width: 701px) {