hash: document that Clone may only return ErrUnsupported or a nil error

hash: document that Clone may only return ErrUnsupported or a nil error

Updates #69521

Change-Id: I6a6a6964ce384a80c4c89efe67c260dc6e9ed6c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/685235
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Austin Clements <austin@google.com>