os/exec: add example for CommandContext

Updates #16360

Change-Id: I0e0afe7a89f2ebcb3e5bbc345f77a605d3afc398
Reviewed-on: https://go-review.googlesource.com/30103
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/exec/example_test.go

@@ -6,13 +6,15 @@ package exec_test
 
 import (
 	"bytes"
+	"context"
 	"encoding/json"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"log"
 	"os/exec"
 	"strings"
+	"time"
 )
 
 func ExampleLookPath() {
@@ -123,3 +125,13 @@ func ExampleCmd_CombinedOutput() {
 	}
 	fmt.Printf("%s\n", stdoutStderr)
 }
+
+func ExampleCommandContext() {
+	ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
+	defer cancel()
+
+	if err := exec.CommandContext(ctx, "sleep", "5").Run(); err != nil {
+		// This will fail after 100 milliseconds. The 5 second sleep
+		// will be interrupted.
+	}
+}