Improve mobile-friendliness of API docs

Removes min-width rule from body so that the page does not scroll
horizontally on mobile displays; hides out-of-band information.

src/librustdoc/html/static/main.css

@@ -64,7 +64,6 @@
 
 body {
     color: #333;
-    min-width: 500px;
     font: 16px/1.4 "Source Serif Pro", "Helvetica Neue", Helvetica, Arial, sans-serif;
     margin: 0;
     position: relative;
@@ -592,6 +591,14 @@ pre.rust { position: relative; }
         margin-left: 0px;
     }
 
+    .content .in-band {
+        width: 100%;
+    }
+
+    .content .out-of-band {
+        display: none;
+    }
+
     .toggle-wrapper > .collapse-toggle {
         left: 0px;
     }