release notes for the breaking change

Author: ktoso

Release Notes/511.md

@@ -67,6 +67,11 @@
 
 ## API-Incompatible Changes
 
+- `MacroDefinition` used for expanding macros:
+  - Description: The `MacroDefinition/expansion` enum case used to have two values (`(MacroExpansionExprSyntax, replacements: [Replacement])`), has now gained another value in order to support generic argument replacements in macro expansions: `(MacroExpansionExprSyntax, replacements: [Replacement], genericReplacements: [GenericArgumentReplacement])`
+  - Pull request: https://github.com/apple/swift-syntax/pull/2450
+  - Migration steps: Code which exhaustively checked over the enum should be changed to `case .expansion(let node, let replacements, let genericReplacements):`. Creating the `.extension` gained a compatibility shim, retaining the previous syntax source compatible (`return .expansion(node, replacements: [])`).
+
 - Effect specifiers:
   - Description: The `unexpectedAfterThrowsSpecifier` node of the various effect specifiers has been removed.
   - Pull request: https://github.com/apple/swift-syntax/pull/2219