bytes,strings: add available godoc link

bytes,strings: add available godoc link

Change-Id: Id9706a783d3321e3706eeee102286522e7968efd
Reviewed-on: https://go-review.googlesource.com/c/go/+/534775
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>