C++: Add comments to {Array,Pointer}StoreNode and arrayStoreStepChi.

cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowPrivate.qll

@@ -244,11 +244,15 @@ private predicate arrayStoreStepChi(Node node1, ArrayContent a, PostUpdateNode n
     node1.asInstruction() = store and
     (
       // `x[i] = taint()`
+      // This matches the characteristic predicate in `ArrayStoreNode`.
       store.getDestinationAddress() instanceof PointerAddInstruction
       or
       // `*p = taint()`
+      // This matches the characteristic predicate in `PointerStoreNode`.
       store.getDestinationAddress().(CopyValueInstruction).getUnary() instanceof LoadInstruction
     ) and
+    // This `ChiInstruction` will always have a non-conflated result because both `ArrayStoreNode`
+    // and `PointerStoreNode` require it in their characteristic predicates.
     node2.asInstruction().(ChiInstruction).getPartial() = store
   )
 }

cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll

@@ -389,6 +389,10 @@ private class ExplicitSingleFieldStoreQualifierNode extends PartialDefinitionNod
   }
 }
 
+/**
+ * The `PostUpdateNode` that is the target of a `arrayStoreStepChi` store step. The overriden
+ * `ChiInstruction` corresponds to the instruction represented by `node2` in `arrayStoreStepChi`.
+ */
 private class ArrayStoreNode extends PartialDefinitionNode {
   override ChiInstruction instr;
   PointerAddInstruction add;
@@ -406,6 +410,10 @@ private class ArrayStoreNode extends PartialDefinitionNode {
   override Expr getDefinedExpr() { result = add.getLeft().getUnconvertedResultExpression() }
 }
 
+/**
+ * The `PostUpdateNode` that is the target of a `arrayStoreStepChi` store step. The overriden
+ * `ChiInstruction` corresponds to the instruction represented by `node2` in `arrayStoreStepChi`.
+ */
 private class PointerStoreNode extends PostUpdateNode {
   override ChiInstruction instr;