spec: clarify when a program exits
spec: clarify when a program exits

Fixes issue 8023.

LGTM=rsc
R=r, iant, ken, rsc
CC=golang-codereviews
https://codereview.appspot.com/98340043

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