go spec: restricted expressions may still be parenthesized
go spec: restricted expressions may still be parenthesized

No language change.

- added a few examples with parentheses
- added a corresponding sentence to assignments
  (this explicitly permits: (_) = 0, currently allowed by 6g,
  gofmt, but marked as an error by gccgo).

R=rsc, r, iant
CC=golang-dev
http://codereview.appspot.com/4446071

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