It's not immediately appaerent that a method must
be used to wrap the handler, so add a basic
example to guide users to the right API.
Fixes #74121
Change-Id: I23fc3dff6fff9bf4eb29c099bc77da8c99620671
Reviewed-on: https://go-review.googlesource.com/c/go/+/681256
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>