doc/go1.18: add documentation for changes to go/types

doc/go1.18: add documentation for changes to go/types

The number of involved CLs is too large (hundreds) so
no CLs are mentioned in (html) comments.

Updates #47694

Change-Id: I655d800a1e56a71e9d70a190f1c42c17baf6861e
Reviewed-on: https://go-review.googlesource.com/c/go/+/369099
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>