slices: update docs for All, Backward, Values

slices: update docs for All, Backward, Values

For #61899

Change-Id: I3586b9b59e87159d21e1a270dabb3af213592739
Reviewed-on: https://go-review.googlesource.com/c/go/+/595515
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>