updated release notes

Author: vinu-vanjari

Release Notes/511.md

@@ -36,6 +36,10 @@
   - Description: `SwiftParser` adds an extension on `String` to check if it can be used as an identifier in a given context.
   - Pull Request: https://github.com/apple/swift-syntax/pull/2434
 
+- `TriviaPiece.isComment`
+  - Description: `TriviaPiece` now has a computed property `isComment` that returns `true` if the trivia piece is a comment.
+  - Pull Request: https://github.com/apple/swift-syntax/pull/2469
+
 ## API Behavior Changes
 
 ## Deprecations