go/types, types2: add additional documentation for Underlying
The concept of an underlying type has become more complicated with the
addition of TypeParam and Alias types. Update the documentation to
clarify that it strips off Named, TypeParam, and Alias types, and to
reference the spec.
Fixes #65774
Change-Id: I40a8efe15b45591b95068acbf4ef9eae17a4cef1
Reviewed-on: https://go-review.googlesource.com/c/go/+/585456
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
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