path/filepath.Rel: document that the returned path is always relative
path/filepath.Rel: document that the returned path is always relative

Fixes issue 2593.

R=rsc, alex.brainman, n13m3y3r
CC=golang-dev
http://codereview.appspot.com/5500052
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go