Also, better documentation on the underlying cause.

Change-Id: I0ee93b6e9f2ada52d9a32a322b77fda783ddf076
Reviewed-on: https://go-review.googlesource.com/c/tools/+/525215
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>