Better documentation for equivocating certs in statemachine tests

Author: tbagrel1

ouroboros-consensus/test/storage-test/Test/Ouroboros/Storage/PerasCertDB/StateMachine.hs

@@ -116,7 +116,10 @@ instance StateModel Model where
     action ->
       model.open && case action of
         CloseDB -> True
-        -- Do not add equivocating certificates.
+        -- Equivocating certificates are not possible by construction,
+        -- so we should ensure that adding a certificate with the same round
+        -- number but different content is rejected.
+        -- See https://tweag.github.io/cardano-peras/peras-design.pdf#subsection.2.2.2
         AddCert cert -> all p model.certs
          where
           p cert' = getPerasCertRound cert /= getPerasCertRound cert' || cert == cert'