encoding/json: document coercion of invalid UTF-8 characters
encoding/json: document coercion of invalid UTF-8 characters

Fixes issue 8342.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/122180043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go