io: document that a Writer must not write to p

Per golang-nuts question. Writing to p breaks
other writers (e.g. io.MultiWriter).

Make this explicit.

LGTM=gri, r, rsc
R=r, rsc, gri, joshlf13
CC=golang-codereviews
https://golang.org/cl/87780046

src/pkg/io/io.go

@@ -74,6 +74,7 @@ type Reader interface {
 // It returns the number of bytes written from p (0 <= n <= len(p))
 // and any error encountered that caused the write to stop early.
 // Write must return a non-nil error if it returns n < len(p).
+// Write must not modify the slice data, even temporarily.
 type Writer interface {
 	Write(p []byte) (n int, err error)
 }