math/bits: add examples for leading zero methods

Change-Id: Ib491d144387a7675af370f7b925fe6e62440d153
Reviewed-on: https://go-review.googlesource.com/48966
Run-TryBot: Kevin Burke <kev@inburke.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Kevin Burke <kev@inburke.com>

src/math/bits/example_test.go

+// Copyright 2017 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 bits_test
+
+import (
+	"fmt"
+	"math/bits"
+)
+
+func ExampleLeadingZeros16() {
+	fmt.Println(bits.LeadingZeros16(0))
+	fmt.Println(bits.LeadingZeros16(1))
+	fmt.Println(bits.LeadingZeros16(256))
+	fmt.Println(bits.LeadingZeros16(65535))
+	// Output:
+	// 16
+	// 15
+	// 7
+	// 0
+}
+
+func ExampleLeadingZeros32() {
+	fmt.Println(bits.LeadingZeros32(0))
+	fmt.Println(bits.LeadingZeros32(1))
+	// Output:
+	// 32
+	// 31
+}
+
+func ExampleLeadingZeros64() {
+	fmt.Println(bits.LeadingZeros64(0))
+	fmt.Println(bits.LeadingZeros64(1))
+	// Output:
+	// 64
+	// 63
+}