os: document that Exit does not run deferred calls
os: document that Exit does not run deferred calls

Fixes issue 4101.

R=dsymonds, rsc
CC=golang-dev
https://codereview.appspot.com/7065048
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go