rustdoc: Highlight methods jumped to

This helps figure out where you actually jumped to, especially if the thing you
jumped to is at the very bottom of the page.

Closes #9905

src/librustdoc/html/static/main.css

@@ -301,3 +301,5 @@ a {
 .stability.Stable { border-color: #AEC516; color: #7c8b10; }
 .stability.Frozen { border-color: #009431; color: #007726; }
 .stability.Locked { border-color: #0084B6; color: #00668c; }
+
+:target { background: #FDFFD3; }