sync: name the Map.CompareAndSwap return value

sync: name the Map.CompareAndSwap return value

The godoc for sync.Map.CompareAndSwap does not document the meaning
of its return value. Document it by giving it a name.

Change-Id: I50ad9c078a7885f5ce83489d66d138d491c35861
Reviewed-on: https://go-review.googlesource.com/c/go/+/572657
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>