Add QLDoc for toString on DataFlowType

Author: owen-mc

go/ql/lib/semmle/go/dataflow/internal/DataFlowPrivate.qll

@@ -228,6 +228,7 @@ private newtype TDataFlowType =
   TTodoDataFlowType2() // Add a dummy value to prevent bad functionality-induced joins arising from a type of size 1.
 
 class DataFlowType extends TDataFlowType {
+  /** Gets a textual representation of this element. */
   string toString() { result = "" }
 }