spec: clarify that break/continue do not work across function boundaries
spec: clarify that break/continue do not work across function boundaries

Also made it extra clear for goto statements (even though label scopes
are already limited to the function defining a label).

Fixes issue 8040.

LGTM=r, rsc
R=r, rsc, iant, ken
CC=golang-codereviews
https://codereview.appspot.com/99550043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang