Cache element docs (and add to completions) (#23694).

pkg/analyzer/lib/src/generated/element_handle.dart

Index: pkg/analyzer/lib/src/generated/element_handle.dart
diff --git a/pkg/analyzer/lib/src/generated/element_handle.dart b/pkg/analyzer/lib/src/generated/element_handle.dart
index f35533be88c9e6d4ec15f845b088fc5c3901edc9..f36d436ce88ceafb157d61e6eb397de8203b13fa 100644
--- a/pkg/analyzer/lib/src/generated/element_handle.dart
+++ b/pkg/analyzer/lib/src/generated/element_handle.dart
@@ -339,6 +339,9 @@ abstract class ElementHandle implements Element {
   SourceRange get docRange => actualElement.docRange;
 
   @override
+  String get documentationComment => actualElement.documentationComment;
+
+  @override
   Element get enclosingElement => actualElement.enclosingElement;
 
   @override
@@ -392,8 +395,7 @@ abstract class ElementHandle implements Element {
   accept(ElementVisitor visitor) => actualElement.accept(visitor);
 
   @override
-  String computeDocumentationComment() =>
-      actualElement.computeDocumentationComment();
+  String computeDocumentationComment() => documentationComment;
 
   @override
   AstNode computeNode() => actualElement.computeNode();