spec: channel operations are restricted by the channel direction

spec: channel operations are restricted by the channel direction

Also:
- 'for' statements with a range clause do not accept send-only
   channels
- '_, _ = range ch' is not equivalent to "_ = range ch" if ch
   is a channel (rewriting the latter to the former leads to
   an invalid range clause).

These clarifications document the status quo.

R=rsc, r, iant, ken
CC=golang-dev
https://golang.org/cl/6874053