[analyzer] Fix documentation for GeneralizingElementVisitor2.

The tree describing the type structure assumed by the visitor was out
of date:

- It included a reference to `PromotableElement`, which no longer
  exists.

- It was missing references to `EnumElement`, `ExtensionElement`,
  `ExtensionTypeElement`, `GenericFunctionTypeElement`,
  `LocalVariableElement`, `MixinElement`, `PropertyAccessorElement`,
  and `TopLevelFunctionElement`.

- It incorrectly listed `FormalParameterElement` under
  `VariableElement`. In fact, `visitFormalParameterElement` redirects
  to `visitElement`.

This is a step towards enabling the `comment_references` lint in the
analyzer codebase.

Change-Id: I6a6a69646cefded105739f207df1554d4beb717d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/450741
Auto-Submit: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: stereotype441

pkg/analyzer/lib/dart/element/visitor2.dart

@@ -47,27 +47,33 @@ import 'package:analyzer/dart/element/element.dart';
 /// <pre>
 /// [Element]
 ///   [ClassElement]
+///   [EnumElement]
 ///   [ExecutableElement]
-///       [ConstructorElement]
+///     [ConstructorElement]
+///     [LocalFunctionElement]
+///     [MethodElement]
+///     [PropertyAccessorElement]
 ///       [GetterElement]
-///       [LocalFunctionElement]
-///       [MethodElement]
 ///       [SetterElement]
+///     [TopLevelFunctionElement]
+///   [ExtensionElement]
+///   [ExtensionTypeElement]
+///   [FormalParameterElement]
+///     [FieldFormalParameterElement]
+///     [SuperFormalParameterElement]
+///   [GenericFunctionTypeElement]
 ///   [LabelElement]
 ///   [LibraryElement]
+///   [MixinElement]
 ///   [MultiplyDefinedElement]
 ///   [PrefixElement]
 ///   [TypeAliasElement]
 ///   [TypeParameterElement]
 ///   [VariableElement]
-///       [PropertyInducingElement]
-///           [FieldElement]
-///           [TopLevelVariableElement]
-///       [PromotableElement]
-///           [LocalVariableElement]
-///           [FormalParameterElement]
-///               [FieldFormalParameterElement]
-///               [SuperFormalParameterElement]
+///     [LocalVariableElement]
+///     [PropertyInducingElement]
+///       [FieldElement]
+///       [TopLevelVariableElement]
 /// </pre>
 ///
 /// Subclasses that override a visit method must either invoke the overridden