io: document non-guarantees of io.Closer
io: document non-guarantees of io.Closer

R=r, golang-dev
CC=golang-dev
https://codereview.appspot.com/8575043

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