add qldoc to isPrivateField

javascript/ql/src/semmle/javascript/dataflow/DataFlow.qll

@@ -536,6 +536,9 @@ module DataFlow {
     pragma[noinline]
     predicate accesses(Node base, string p) { getBase() = base and getPropertyName() = p }
     
+    /**
+     * Holds if this data flow node reads or writes a private field in a class.
+     */ 
     predicate isPrivateField() {
       getPropertyName().charAt(0) = "#" and getPropertyNameExpr() instanceof Label
     }