Revert "spec: document the underlying type of comparable"

This reverts CL 380854.

Per the conluding discussions on #50791. A follow-up will
document `comparable` more thoroughly.

For #50791.

Change-Id: I15db9051784a012f713e28d725c3b8bbfeb40569
Reviewed-on: https://go-review.googlesource.com/c/go/+/381076


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>