The current Error documentation is vacuous and doesn't say anything
about what this interface is actually for. Expand to include its meaning
and why it might be used.
Change-Id: I6a6a636cbd5f5788cb9d1a88845de16b98f7424b
Reviewed-on: https://go-review.googlesource.com/c/go/+/670635 Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>