move comment on StringHeader to fix godoc output.

R=rsc, ken2
CC=golang-dev
https://golang.org/cl/196055

src/pkg/reflect/value.go

@@ -281,12 +281,13 @@ func (v *Int64Value) Set(x int64) {
 // Set sets v to the value x.
 func (v *Int64Value) SetValue(x Value) { v.Set(x.(*Int64Value).Get()) }
 
-// StringValue represents a string value.
-// runtime representation of slice
+// StringHeader is the runtime representation of a string.
 type StringHeader struct {
 	Data uintptr
 	Len  int
 }
+
+// StringValue represents a string value.
 type StringValue struct {
 	value
 }