os: fix docs for Expand there is no concept of “undefined” variables for Expand。
os: fix docs for Expand
there is no concept of “undefined” variables for Expand。

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/6946063
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go