doc/faq: add question about converting from []T to []interface{}
doc/faq: add question about converting from []T to []interface{}

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/4639046

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