docs(poll): describe response retirement contract

README.md

@@ -116,6 +116,13 @@ submission. Option removal and option changes invalidate only answers bound to
 that option, and repeated removal/revocation is an idempotent replay even after
 the Poll has moved out of an editable lifecycle state.
 
+Owning modules can also retire a participant's responses through the optional
+response-retirement extension. Retirement is idempotent and soft-deletes the
+live rows so aggregation and capacity checks stop counting them, while answers
+and a reason/source retirement record remain available for audit. The boundary
+accepts only server-trusted respondent or invitation identities and rejects
+secret-like metadata.
+
 ## Identified response invariant
 
 Poll stores at most one active response for each identified respondent in a