effective go: explain about values/pointers in String() example
effective go: explain about values/pointers in String() example

Fixes issue 1796.

R=rsc, r2, niemeyer
CC=golang-dev
http://codereview.appspot.com/4539049

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