sync: add caution about where to call (*WaitGroup).Add
sync: add caution about where to call (*WaitGroup).Add

Fixes issue 4762.

R=daniel.morsing, adg
CC=golang-dev
https://codereview.appspot.com/7308045

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