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 #6724.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/100430043