context: cancel the context in ExampleWithTimeout, with explanation

Fixes #16230

Change-Id: Ibb10234a6c3ab8bd0cfd93c2ebe8cfa66f80f6b0
Reviewed-on: https://go-review.googlesource.com/24682Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/context/withtimeout_test.go

@@ -13,13 +13,21 @@ import (
 func ExampleWithTimeout() {
 	// Pass a context with a timeout to tell a blocking function that it
 	// should abandon its work after the timeout elapses.
-	ctx, _ := context.WithTimeout(context.Background(), 50*time.Millisecond)
+	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
+
 	select {
 	case <-time.After(1 * time.Second):
 		fmt.Println("overslept")
 	case <-ctx.Done():
 		fmt.Println(ctx.Err()) // prints "context deadline exceeded"
 	}
+
+	// Even though ctx should have expired already, it is good
+	// practice to call its cancelation function in any case.
+	// Failure to do so may keep the context and its parent alive
+	// longer than necessary.
+	cancel()
+
 	// Output:
 	// context deadline exceeded
 }