exec: add a little package commentary explaining its relationship to os.Process.
exec: add a little package commentary explaining its
relationship to os.Process.

R=golang-dev, rsc1
CC=golang-dev
http://codereview.appspot.com/4384041
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go