go/types,types2: revert documentation for Type.Underlying

In the dev.typeparams branch, the documentation for Type.Underlying was
updated with commentary about forwarding chains. This aspect of
Underlying should not be exposed to the user. Revert to the
documentation of Go 1.16.

Fixes #51036

Change-Id: I4b73d3908a88606314aab56540cca91c014dc426
Reviewed-on: https://go-review.googlesource.com/c/go/+/388036


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>