io: document that i/o is not necessarily safe for parallel access.
io: document that i/o is not necessarily safe for parallel access.

Updates issue 1599.

R=golang-dev, adg, dsymonds
CC=golang-dev
http://codereview.appspot.com/5704052

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go