spec: several clarifications to language on channels
spec: several clarifications to language on channels

- A channel may be used between any number of goroutines,
  not just two.
- Replace "passing a value" (which is not further defined)
  by "sending and receiving a value".
- Made syntax production more symmetric.
- Talk about unbuffered channels before buffered channels.
- Clarify what the comma,ok receive values mean (issue 7785).

Not a language change.

Fixes issue 7785.

LGTM=rsc, r, iant
R=r, rsc, iant, ken
CC=golang-codereviews
https://codereview.appspot.com/94030045

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