encoding/json: clarify documention for Unmarshal into a pointer.
encoding/json: clarify documention for Unmarshal into a pointer.

Fixes #18730.

Change-Id: If3ef28e62f7e449d4c8dc1dfd78f7d6f5a87ed26
Reviewed-on: https://go-review.googlesource.com/36478
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

coreos/go-json