io: explain what (0,nil) means from Read Also add a new variable ErrNoProgress that io.Readers can use to report ineffectual Read calls. Fixes issue 5310.
io: explain what (0,nil) means from Read
Also add a new variable ErrNoProgress that io.Readers can use to
report ineffectual Read calls.
Fixes issue 5310.

R=golang-dev, dsymonds, bradfitz
CC=golang-dev
https://codereview.appspot.com/8845043

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