reflect: document the unreliability of StringHeader and SliceHeader

R=golang-dev, adg, dvyukov
CC=golang-dev
https://golang.org/cl/8494045

src/pkg/reflect/value.go

@@ -1699,14 +1699,22 @@ func (v Value) UnsafeAddr() uintptr {
 }
 
 // StringHeader is the runtime representation of a string.
-// It cannot be used safely or portably.
+// It cannot be used safely or portably and its representation may
+// change in a later release.
+// Moreover, the Data field is not sufficient to guarantee the data
+// it references will not be garbage collected, so programs must keep
+// a separate, correctly typed pointer to the underlying data.
 type StringHeader struct {
 	Data uintptr
 	Len  int
 }
 
 // SliceHeader is the runtime representation of a slice.
-// It cannot be used safely or portably.
+// It cannot be used safely or portably and its representation may
+// change in a later release.
+// Moreover, the Data field is not sufficient to guarantee the data
+// it references will not be garbage collected, so programs must keep
+// a separate, correctly typed pointer to the underlying data.
 type SliceHeader struct {
 	Data uintptr
 	Len  int