io/ioutil: document EOF behavior in ReadFile and ReadAll
io/ioutil: document EOF behavior in ReadFile and ReadAll

Fixes issue 2862.

R=golang-dev, n13m3y3r, iant
CC=golang-dev
http://codereview.appspot.com/5646048
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go