hash: mention the new Cloner interface in Hash docs.

hash: mention the new Cloner interface in Hash docs.

We mention that already in Cloner docs, but to be consistent, also
mention that in Hash.

Change-Id: Iee33d545662b7054973666bd45998a37f3037a51
Reviewed-on: https://go-review.googlesource.com/c/go/+/675915
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: David Chase <drchase@google.com>