log: adds a Logger Output method Example

Change-Id: Ia3e351169a4ebe6db5e5f37b668f23dc8c992c78
Reviewed-on: https://go-review.googlesource.com/48877
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/log/example_test.go

@@ -11,11 +11,31 @@ import (
 )
 
 func ExampleLogger() {
-	var buf bytes.Buffer
-	logger := log.New(&buf, "logger: ", log.Lshortfile)
+	var (
+		buf    bytes.Buffer
+		logger = log.New(&buf, "logger: ", log.Lshortfile)
+	)
+
 	logger.Print("Hello, log file!")
 
 	fmt.Print(&buf)
 	// Output:
-	// logger: example_test.go:16: Hello, log file!
+	// logger: example_test.go:19: Hello, log file!
+}
+
+func ExampleLogger_Output() {
+	var (
+		buf    bytes.Buffer
+		logger = log.New(&buf, "INFO: ", log.Lshortfile)
+
+		infof = func(info string) {
+			logger.Output(2, info)
+		}
+	)
+
+	infof("Hello world")
+
+	fmt.Print(&buf)
+	// Output:
+	// INFO: example_test.go:36: Hello world
 }