rustdoc: add tooltip to resize track

Author: notriddle

src/librustdoc/html/templates/page.html

@@ -114,7 +114,7 @@ <h2>Files</h2> {# #}
         {% endif %}
         {{ sidebar|safe }}
     </nav> {# #}
-    <div class="sidebar-resizer"></div> {# #}
+    <div class="sidebar-resizer" title="Drag to resize sidebar"></div> {# #}
     <main>
         {% if page.css_class != "src" %}<div class="width-limiter">{% endif %}
             {# defined in storage.js to avoid duplicating complex UI across every page #}