Docs: Add unit test guidance for uncertain claims

When complexity claims cannot be verified from documentation or source
code, write unit tests to measure timing and confirm the complexity class.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: heikkitoivonen

CORRECTNESS_REVIEW_PROMPT.md

@@ -14,6 +14,10 @@ You are an expert Python core contributor. You must review the time and space co
 
 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.
 
+## When Unsure
+
+If you cannot verify a complexity claim from documentation or source code, write unit tests to measure timing across different input sizes and confirm the claimed complexity class.
+
 ## Output
 
 Report any errors with: