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>