Docs: Require release branches for CPython source code reviews

Co-Authored-By: Amp <amp@ampcode.com>

Author: heikkitoivonen

CORRECTNESS_REVIEW_PROMPT.md

@@ -10,6 +10,10 @@ You are an expert Python core contributor. You must review the time and space co
 - Validate code examples produce expected results
 - Flag any misleading or incomplete explanations
 
+## Source Code References
+
+When reviewing against CPython source code, you MUST use the corresponding release branch (e.g., `3.14`, `3.13`), never `main`. The `main` branch contains unreleased changes that may not reflect documented behavior.
+
 ## Output
 
 Report any errors with: