C#: Address more review comments.

Author: aschackmull

csharp/ql/lib/semmle/code/csharp/controlflow/ControlFlowElement.qll

@@ -40,6 +40,7 @@ class ControlFlowElement extends ExprOrStmtParent, @control_flow_element {
    */
   Nodes::ElementNode getAControlFlowNode() { result.getAstNode() = this }
 
+  /** Gets the control flow node for this element. */
   ControlFlow::Node getControlFlowNode() { result.getAstNode() = this }
 
   /** Gets the basic block in which this element occurs. */

csharp/ql/lib/semmle/code/csharp/dataflow/Nullness.qll

@@ -34,7 +34,7 @@ private Expr maybeNullExpr(Expr reason) {
   or
   result.(AssignExpr).getRValue() = maybeNullExpr(reason)
   or
-  result.(Cast).getExpr() = maybeNullExpr(reason)
+  result.(CastExpr).getExpr() = maybeNullExpr(reason)
   or
   result =
     any(ConditionalExpr ce |