reflect: document that Value.Slice panics on an unaddressable array.
reflect: document that Value.Slice panics on an unaddressable array.

Fixes issue 4736.

R=rsc
CC=golang-dev
https://codereview.appspot.com/7239045

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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