add populated search box to docs 404 page

reference https://github.com/rust-lang/rust/issues/18498

src/doc/not_found.md

@@ -11,9 +11,33 @@ Looks like you've taken a wrong turn.
 
 Some things that might be helpful to you though:
 
+## Search
+* <form action="https://duckduckgo.com/">
+    <input type="text" id="code.search" name="q" size="80"></input>
+    <input type="submit" value="Search DuckDuckGo">
+</form>
+
 ## Reference
 * [The Rust official site](http://rust-lang.org)
 * [The Rust reference](http://doc.rust-lang.org/reference.html) (* [PDF](http://doc.rust-lang.org/reference.pdf))
 
 ## Docs
 * [The standard library](http://doc.rust-lang.org/std/)
+
+<script>
+function populate_search_box() {
+
+    var last = document.URL.split("/").pop();
+    var tokens = last.split(".");
+    var op = [];
+    for (var i=0; i < tokens.length; i++) {
+        if (tokens[i].indexOf("#") == -1) 
+            op.push(tokens[i]);
+    }
+
+    var search = document.getElementById('code.search');
+    search.value = op.join(' ') + " site:doc.rust-lang.org";
+}
+populate_search_box();
+</script>
+