html: add examples to the functions

Change-Id: I129d70304ae4e4694d9217826b18b341e3834d3c
Reviewed-on: https://go-review.googlesource.com/11201Reviewed-by: Andrew Gerrand <adg@golang.org>

src/html/example_test.go

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package html_test
+
+import (
+	"fmt"
+	"html"
+)
+
+func ExampleEscapeString() {
+	const s = `"Fran & Freddie's Diner" <tasty@example.com>`
+	fmt.Println(html.EscapeString(s))
+	// Output: &#34;Fran &amp; Freddie&#39;s Diner&#34; &lt;tasty@example.com&gt;
+}
+
+func ExampleUnescapeString() {
+	const s = `&quot;Fran &amp; Freddie&#39;s Diner&quot; &lt;tasty@example.com&gt;`
+	fmt.Println(html.UnescapeString(s))
+	// Output: "Fran & Freddie's Diner" <tasty@example.com>
+}