os: document FileInfo.Size as system-dependent for irregular files
os: document FileInfo.Size as system-dependent for irregular files

R=rsc, r
CC=golang-dev
http://codereview.appspot.com/5795059
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go