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

pkg/analyzer/lib/src/dart/element/member.dart

Index: pkg/analyzer/lib/src/dart/element/member.dart
diff --git a/pkg/analyzer/lib/src/dart/element/member.dart b/pkg/analyzer/lib/src/dart/element/member.dart
index ef22fa9a9c2b2916a4c394b47ab14319df6559d6..fea415f5d4ba75826035cac2278ac9701b259e38 100644
--- a/pkg/analyzer/lib/src/dart/element/member.dart
+++ b/pkg/analyzer/lib/src/dart/element/member.dart
@@ -442,6 +442,9 @@ abstract class Member implements Element {
   @override
   SourceRange get docRange => _baseElement.docRange;
 
+  @override
+  String get documentationComment => _baseElement.documentationComment;
+
   int get id => _baseElement.id;
 
   @override
@@ -487,8 +490,7 @@ abstract class Member implements Element {
   CompilationUnit get unit => _baseElement.unit;
 
   @override
-  String computeDocumentationComment() =>
-      _baseElement.computeDocumentationComment();
+  String computeDocumentationComment() => documentationComment;
 
   @override
   AstNode computeNode() => _baseElement.computeNode();