Swift: Respond to PR comments.

Author: MathiasVP

swift/ql/lib/codeql/swift/controlflow/internal/Scope.qll

@@ -28,11 +28,19 @@ class Scope extends AstNode instanceof Scope::Range {
 
 cached
 private module Cached {
-  private AstNode getParentOfAstStep(Element e) {
+  /**
+   * Gets the immediate parent of a non-`AstNode` element `e`.
+   *
+   * We restrict `e` to be a non-`AstNode` to skip past non-`AstNode` in
+   * the transitive closure computation in `getParentOfAst`. This is
+   * necessary because the parent of an `AstNode` is not necessarily an `AstNode`.
+   */
+  private Element getParentOfAstStep(Element e) {
     not e instanceof AstNode and
     result = getImmediateParent(e)
   }
 
+  /** Gets the nearest enclosing parent of `ast` that is an `AstNode`. */
   cached
   AstNode getParentOfAst(AstNode ast) { result = getParentOfAstStep*(getImmediateParent(ast)) }
 }