reflect: add comment about Type.Field allocation

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/5586044

src/pkg/reflect/type.go

@@ -789,6 +789,14 @@ func (t *structType) Field(i int) (f StructField) {
 		f.Tag = StructTag(*p.tag)
 	}
 	f.Offset = p.offset
+
+	// NOTE(rsc): This is the only allocation in the interface
+	// presented by a reflect.Type.  It would be nice to avoid,
+	// at least in the common cases, but we need to make sure
+	// that misbehaving clients of reflect cannot affect other
+	// uses of reflect.  One possibility is CL 5371098, but we
+	// postponed that ugliness until there is a demonstrated
+	// need for the performance.  This is issue 2320.
 	f.Index = []int{i}
 	return
 }