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

Updates #1599.

R=golang-dev, adg, dsymonds
CC=golang-dev
https://golang.org/cl/5704052

src/pkg/io/io.go

@@ -6,6 +6,10 @@
 // Its primary job is to wrap existing implementations of such primitives,
 // such as those in package os, into shared public interfaces that
 // abstract the functionality, plus some other related primitives.
+//
+// Because these interfaces and primitives wrap lower-level operations with
+// various implementations, unless otherwise informed clients should not
+// assume they are safe for parallel execution.
 package io
 
 import (