cmd/api: internal debugging supprt
cmd/api: internal debugging supprt

Document that the package cache has
an issue (8425) to speed up future
debugging.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/143980043

wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old