rustdoc: clean up docblock table CSS

* The rule `display: block` had no noticeable effect. Technically, because
  markdown tables have a tbody and thead, they get wrapped in an [anonymous
  table box] in the CSS tree, nested within the `<table>` element's block
  layout box.

  This rule was added in rust-lang#87230 to make the table side-scrolling, but
  this same issue was doubly fixed in rust-lang#88742 by wrapping it in an explicit
  `<div>` tag. Since accessibility advocates recommend the wrapper div over
  marking the table as `display: block`, we'll stick with that.

  https://adrianroselli.com/2020/11/under-engineered-responsive-tables.html

* The rule `width: calc(100% - 2px)` had no visible effect, because the
  anonymous table box was not affected.

* The style is tweaked to basically be the same style GitHub uses.
  In particular, it adds zebra stripes, and removes dotted borders.

[anonymous table box]: https://www.w3.org/TR/CSS2/tables.html#anonymous-boxes

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

@@ -631,22 +631,16 @@ pre, .rustdoc.source .example-wrap {
 
 .docblock table {
 	margin: .5em 0;
-	width: calc(100% - 2px);
-	overflow-x: auto;
-	display: block;
 	border-collapse: collapse;
 }
 
-.docblock table td {
+.docblock table td, .docblock table th {
 	padding: .5em;
-	border: 1px dashed var(--border-color);
-	vertical-align: top;
+	border: 1px solid var(--border-color);
 }
 
-.docblock table th {
-	padding: .5em;
-	text-align: left;
-	border: 1px solid var(--border-color);
+.docblock table tr:nth-child(2n) {
+	background: var(--table-alt-row-background-color);
 }
 
 /* Shift "where ..." part of method or fn definition down a line */

src/librustdoc/html/static/css/themes/ayu.css

@@ -87,6 +87,7 @@ Original by Dempfi (https://github.com/dempfi/ayu)
 	--crate-search-hover-border: #e0e0e0;
 	--source-sidebar-background-selected: #14191f;
 	--source-sidebar-background-hover: #14191f;
+	--table-alt-row-background-color: #191f26;
 }
 
 h1, h2, h3, h4 {

src/librustdoc/html/static/css/themes/dark.css

@@ -82,6 +82,7 @@
 	--crate-search-hover-border: #2196f3;
 	--source-sidebar-background-selected: #333;
 	--source-sidebar-background-hover: #444;
+	--table-alt-row-background-color: #2A2A2A;
 }
 
 .content .item-info::before { color: #ccc; }

src/librustdoc/html/static/css/themes/light.css

@@ -79,6 +79,7 @@
 	--crate-search-hover-border: #717171;
 	--source-sidebar-background-selected: #fff;
 	--source-sidebar-background-hover: #e0e0e0;
+	--table-alt-row-background-color: #F5F5F5;
 }
 
 .content .item-info::before { color: #ccc; }