go/types, types2: clarify a comment and add an extra test

go/types, types2: clarify a comment and add an extra test

Confirm that the current implementation of core type unification
looks correct and update the respective comment. Add an extra test.

Fixes #51376.

Change-Id: I6a603a4baeee2ede5bb4a1d60766204a808936d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/388294
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>