Add the doc comment from the proposal, lightly edited.
The edits are to drop mention of value-error Seq2 usage
and to adjust for the bool result changes.
Fixes #61897.
Change-Id: Iaffbaa301064b2f8739956d602ca5fa23c89a269
Reviewed-on: https://go-review.googlesource.com/c/go/+/591096
Auto-Submit: Russ Cox <rsc@golang.org> Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>