rustdoc: on mobile, make the sidebar full width and linewrap

this is because the mobile sidebar cannot be resized,
unlike on desktop.

Author: lolbinarycat

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

@@ -2506,7 +2506,10 @@ in src-script.js and main.js
 		z-index: 11;
 		/* Reduce height slightly to account for mobile topbar. */
 		height: calc(100vh - 45px);
-		width: 200px;
+		width: 100%;
+	}
+	.sidebar-elems .block li a {
+		white-space: wrap;
 	}
 
 	/* The source view uses a different design for the sidebar toggle, and doesn't have a topbar,