math/cmplx: add examples for Abs, Exp, Polar

Updates #16360

Change-Id: I941519981ff5bda3a113e14fa6be718eb4d2bf83
Reviewed-on: https://go-review.googlesource.com/30554
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/cmplx/example_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 cmplx_test
+
+import (
+	"fmt"
+	"math"
+	"math/cmplx"
+)
+
+func ExampleAbs() {
+	fmt.Printf("%.1f", cmplx.Abs(3+4i))
+	// Output: 5.0
+}
+
+// ExampleExp computes Euler's identity.
+func ExampleExp() {
+	fmt.Printf("%.1f", cmplx.Exp(1i*math.Pi)+1)
+	// Output: (0.0+0.0i)
+}
+
+func ExamplePolar() {
+	r, theta := cmplx.Polar(2i)
+	fmt.Printf("r: %.1f, θ: %.1f*π", r, theta/math.Pi)
+	// Output: r: 2.0, θ: 0.5*π
+}