Add Release notes for base specific SyntaxEnum

Author: rintaro

Release Notes/511.md

@@ -16,6 +16,10 @@
   - Issue: https://github.com/apple/swift-syntax/issues/2015
   - Pull Request: https://github.com/apple/swift-syntax/pull/2021
 
+- `DeclSyntaxEnum`, `StmtSyntaxEnum`, `ExprSyntaxEnum`, `TypeSyntaxEnum`, and `PatternSyntaxEnum`
+  - Description: Enum to exhaustively switch over all different syntax nodes of each base type.
+  - Pull Request: https://github.com/apple/swift-syntax/pull/2351
+
 ## API Behavior Changes
 
 ## Deprecations