errors: mention Is/As in Join docs

Fixes golang#74461

Change-Id: Ic1b1b2b43d59dd67f31c2388ca88650ee847ba93
Reviewed-on: https://go-review.googlesource.com/c/go/+/696737
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>

Author: seankhliao

src/errors/join.go

@@ -16,6 +16,7 @@ import (
 // between each string.
 //
 // A non-nil error returned by Join implements the Unwrap() []error method.
+// The errors may be inspected with [Is] and [As].
 func Join(errs ...error) error {
 	n := 0
 	for _, err := range errs {