Add release note for WithOptionalCodeBlock

Author: DougGregor

Release Notes/511.md

@@ -20,6 +20,10 @@
   - Description: Enum to exhaustively switch over all different syntax nodes of each base type.
   - Pull Request: https://github.com/apple/swift-syntax/pull/2351
 
+- `WithOptionalCodeBlock`
+  - Description: A trait for syntax nodes that have an optional code block, such as `FunctionDeclSyntax` and `InitializerDeclSyntax`.
+  - Pull Request: https://github.com/apple/swift-syntax/pull/2359
+
 ## API Behavior Changes
 
 ## Deprecations