runtime: Added usage example for the runtime.AddCleanup() function.

The existing description of the function lacks usage examples, which makes it difficult to understand, so I added one.

There is no open issue about this, since the implementation seems trivial.

Change-Id: I96b29f0b21d1c7fda04128239633c8a2fc36fef2
Reviewed-on: https://go-review.googlesource.com/c/go/+/649995
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/example_test.go

@@ -6,6 +6,7 @@ package runtime_test
 
 import (
 	"fmt"
+	"os"
 	"runtime"
 	"strings"
 )
@@ -59,3 +60,36 @@ func ExampleFrames() {
 	// - more:true | runtime_test.ExampleFrames.func3
 	// - more:true | runtime_test.ExampleFrames
 }
+
+func ExampleAddCleanup() {
+	tempFile, err := os.CreateTemp(os.TempDir(), "file.*")
+	if err != nil {
+		fmt.Println("failed to create temp file:", err)
+		return
+	}
+
+	ch := make(chan struct{})
+
+	// Attach a cleanup function to the file object.
+	runtime.AddCleanup(&tempFile, func(fileName string) {
+		if err := os.Remove(fileName); err == nil {
+			fmt.Println("temp file has been removed")
+		}
+		ch <- struct{}{}
+	}, tempFile.Name())
+
+	if err := tempFile.Close(); err != nil {
+		fmt.Println("failed to close temp file:", err)
+		return
+	}
+
+	// Run the garbage collector to reclaim unreachable objects
+	// and enqueue their cleanup functions.
+	runtime.GC()
+
+	// Wait until cleanup function is done.
+	<-ch
+
+	// Output:
+	// temp file has been removed
+}