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>

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