A+C: Caleb Spare (individual CLA)

Generated by addca.

R=gobot
CC=golang-dev
https://codereview.appspot.com/7070046
encoding/json: documented that ,string applies only to string and numeric fields.

Fixes issue 4437.

R=golang-dev, adg
CC=golang-dev, rsc
https://codereview.appspot.com/7064045

Committer: Andrew Gerrand <adg@golang.org>