sync: add example for Pool

It was a little tricky to figure out how to go from the documentation
to figuring out the best way to implement a Pool, so I thought I'd
try to provide a simple example. The implementation is mostly taken
from the fmt package.

I'm not happy with the verbosity of the calls to WriteString() etc,
but I wanted to provide a non-trivial example.

Change-Id: Id33a8b6cbf8eb278f71e1f78e20205b436578606
Reviewed-on: https://go-review.googlesource.com/24371Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/sync/example_pool_test.go

+// Copyright 2016 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 sync_test
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"sync"
+	"time"
+)
+
+var bufPool = sync.Pool{
+	New: func() interface{} {
+		// The Pool's New function should generally only return pointer
+		// types, since a pointer can be put into the return interface
+		// value without an allocation:
+		return new(bytes.Buffer)
+	},
+}
+
+// timeNow is a fake version of time.Now for tests.
+func timeNow() time.Time {
+	return time.Unix(1136214245, 0)
+}
+
+func Log(w io.Writer, key, val string) {
+	b := bufPool.Get().(*bytes.Buffer)
+	b.Reset()
+	// Replace this with time.Now() in a real logger.
+	b.WriteString(timeNow().UTC().Format(time.RFC3339))
+	b.WriteByte(' ')
+	b.WriteString(key)
+	b.WriteByte('=')
+	b.WriteString(val)
+	w.Write(b.Bytes())
+	bufPool.Put(b)
+}
+
+func ExamplePool() {
+	Log(os.Stdout, "path", "/search?q=flowers")
+	// Output: 2006-01-02T15:04:05Z path=/search?q=flowers
+}