encoding/gob: document that structs only encode/decode exported fields
encoding/gob: document that structs only encode/decode exported fields

Fixes issue 4579.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7000051

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang