errors: document that the target of Is must be comparable

errors: document that the target of Is must be comparable

If target is not comparable, then errors.Is(err, target) can panic.
(Put another way, if target == target panics, then Is can panic.)

Document that the target must be comparable.

For #74488

Change-Id: I694dc4c91a608b80f044f06dd1c6ac32b8e77c9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/715440
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
TryBot-Bypass: Damien Neil <dneil@google.com>