Update dartdoc to match v4.0.0

Release notes: https://github.com/dart-lang/dartdoc/releases/tag/v4.0.0

Change-Id: I219ba57cdcd3090b301f40807e5d12256f4e32dc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/215485
Reviewed-by: Janice Collins <jcollins@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Janice Collins <jcollins@google.com>

DEPS

@@ -106,7 +106,7 @@ vars = {
   # For more details, see https://github.com/dart-lang/sdk/issues/30164
   "dart_style_rev": "14d9b6fd58cc4744676c12be3cc5eee2a779db82",
 
-  "dartdoc_rev" : "e5ebb7a6e88427db25c21811dc91190475934b17",
+  "dartdoc_rev" : "c2f284f09bcc49025fe26e86b2d45e1b546f81a3",
   "devtools_rev" : "2b47d9ed486479153ca2fd038000950674ed1beb",
   "jsshell_tag": "version:88.0",
   "ffi_rev": "4dd32429880a57b64edaf54c9d5af8a9fa9a4ffb",