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

gnanderson/go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,apparentlymart/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,apparentlymart/golang,gnanderson/go,dsriram/golang,tv42/old-go,gnanderson/go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go