Release notes for SPI MacroExpansion/PluginMessageHandling

(cherry picked from commit 31ea421)

Author: rintaro

Release Notes/600.md

@@ -172,6 +172,11 @@
   - Pull request: https://github.com/apple/swift-syntax/pull/2531
   - Migration steps: Use `if case .backslash = triviaPiece` instead
 
+- All symbols in `SwiftCompilerPluginMessageHandling` are now SPI
+  - Description: This module is only intended to be used from some internal components. Any other modules should not use them directly.
+  - Pull request: https://github.com/apple/swift-syntax/pull/2489
+  - Migration steps: Stop using this module.
+
 ## Template
 
 - *Affected API or two word description*