Add note about type signature searching to docs

src/librustdoc/html/layout.rs

@@ -111,6 +111,10 @@ pub fn render<T: fmt::Display, S: fmt::Display>(
                 <code>trait</code>, <code>typedef</code> (or
                 <code>tdef</code>).
             </p>
+            <p>
+                Search functions by type signature (e.g.
+                <code>vec -> usize</code>)
+            </p>
         </div>
     </div>