encoding/json: document what unmarshal of `null` into non-reference type does
encoding/json: document what unmarshal of `null` into non-reference type does

Originally it was an error, which made perfect sense, but in issue 2540
I got talked out of this sensible behavior. I'm not thrilled with the "new"
behavior but it's been there since Go 1.1 so we're stuck with it now.

Fixes issue 6724.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/100430043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang