doc: expand code.html to discuss the go tool in more depth
doc: expand code.html to discuss the go tool in more depth

R=golang-dev, balasanjay, r
CC=golang-dev
http://codereview.appspot.com/5640045

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