exp/html: update package docs and add an example; a node's children is

a linked list, not a slice.

R=r, minux.ma
CC=golang-dev
https://golang.org/cl/6618055

src/pkg/exp/html/doc.go

@@ -84,7 +84,7 @@ example, to process each anchor node in depth-first order:
 		if n.Type == html.ElementNode && n.Data == "a" {
 			// Do something with n...
 		}
-		for _, c := range n.Child {
+		for c := n.FirstChild; c != nil; c = c.NextSibling {
 			f(c)
 		}
 	}

src/pkg/exp/html/example_test.go

+// Copyright 2012 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.
+
+// This example demonstrates parsing HTML data and walking the resulting tree.
+package html_test
+
+import (
+	"exp/html"
+	"fmt"
+	"log"
+	"strings"
+)
+
+func ExampleParse() {
+	s := `<p>Links:</p><ul><li><a href="foo">Foo</a><li><a href="/bar/baz">BarBaz</a></ul>`
+	doc, err := html.Parse(strings.NewReader(s))
+	if err != nil {
+		log.Fatal(err)
+	}
+	var f func(*html.Node)
+	f = func(n *html.Node) {
+		if n.Type == html.ElementNode && n.Data == "a" {
+			for _, a := range n.Attr {
+				if a.Key == "href" {
+					fmt.Println(a.Val)
+					break
+				}
+			}
+		}
+		for c := n.FirstChild; c != nil; c = c.NextSibling {
+			f(c)
+		}
+	}
+	f(doc)
+	// Output:
+	// foo
+	// /bar/baz
+}