spec: update section on type unification for Go 1.21
This leaves the specific unification details out in favor
of a (forthcoming) section in an appendix.
Change-Id: If984c48bdf71c278e1a2759f9a18c51ef58df999
Reviewed-on: https://go-review.googlesource.com/c/go/+/507417 Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com>