Swift: Add comments.

Author: geoffw0

swift/ql/lib/codeql/swift/dataflow/internal/DataFlowPrivate.qll

@@ -500,19 +500,22 @@ predicate jumpStep(Node pred, Node succ) {
 }
 
 predicate storeStep(Node node1, ContentSet c, Node node2) {
+  // assignment to a member variable `obj.member = value`
   exists(MemberRefExpr ref, AssignExpr assign |
     ref = assign.getDest() and
     node1.asExpr() = assign.getSource() and
     node2.(PostUpdateNode).getPreUpdateNode().asExpr() = ref.getBase() and
     c.isSingleton(any(Content::FieldContent ct | ct.getField() = ref.getMember()))
   )
   or
+  // creation of a tuple `(v1, v2)`
   exists(TupleExpr tuple, int pos |
     node1.asExpr() = tuple.getElement(pos) and
     node2.asExpr() = tuple and
     c.isSingleton(any(Content::TupleContent ct | ct.getIndex() = pos))
   )
   or
+  // assignment to a tuple member `tuple.index = value`
   exists(TupleElementExpr tuple, AssignExpr assign |
     tuple = assign.getDest() and
     node1.asExpr() = assign.getSource() and
@@ -526,13 +529,15 @@ predicate storeStep(Node node1, ContentSet c, Node node2) {
 predicate isLValue(Expr e) { any(AssignExpr assign).getDest() = e }
 
 predicate readStep(Node node1, ContentSet c, Node node2) {
+  // read of a member variable `obj.member`
   exists(MemberRefExpr ref |
     not isLValue(ref) and
     node1.asExpr() = ref.getBase() and
     node2.asExpr() = ref and
     c.isSingleton(any(Content::FieldContent ct | ct.getField() = ref.getMember()))
   )
   or
+  // read of a tuple member `tuple.index`
   exists(TupleElementExpr tuple |
     node1.asExpr() = tuple.getSubExpr() and
     node2.asExpr() = tuple and