io: document non-guarantees of io.Closer

R=r, golang-dev
CC=golang-dev
https://golang.org/cl/8575043

src/pkg/io/io.go

@@ -70,6 +70,9 @@ type Writer interface {
 }
 
 // Closer is the interface that wraps the basic Close method.
+//
+// The behavior of Close after the first call is undefined.
+// Specific implementations may document their own behavior.
 type Closer interface {
 	Close() error
 }