compress/zlib: mention that NewReaderDict can return ErrDictionary
compress/zlib: mention that NewReaderDict can return ErrDictionary

Fixes issue 7935.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, ruiu
https://codereview.appspot.com/147390043

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