doc/go1.2: link to cgo documentation
doc/go1.2: link to cgo documentation

I know it's linked in the previous sentence, but this new link is where I want it to be while reading this sentence.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/21770043

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