review poka

complete comment

pallets/certification/src/lib.rs

@@ -409,7 +409,8 @@ pub mod pallet {
         }
 
         /// perform cert renewal if exisiting, else error with CertDoesNotExist
-        // must be transactional
+        // must be used in transactional context
+        // (it can fail if certification does not exist after having modified state)
         fn try_renew_cert(
             block_number: T::BlockNumber,
             issuer: T::IdtyIndex,