spec: clarify receive operator
spec: clarify receive operator

- receiving from a closed channel returns immediately
- in the ,ok form, the 2nd result is of type bool, not
  just boolean (gc and ggcgo agree).

Per dsymonds' suggestion.

R=r, rsc, ken, iant, dsymonds
CC=golang-dev
http://codereview.appspot.com/6333057

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