clarify definition of tx sender as membership keeper

standards/core/rln-contract.md

@@ -45,7 +45,12 @@ The contract MUST provide the following functionalities:
 - withdraw a deposit.
 
 A membership _holder_ is the entity that controls the secret associated with the respective RLN commitment.
-A membership _keeper_ is the entity that controls the Ethereum address used to register that membership.
+A membership _keeper_ is the sender of the transaction that registered that membership.
+Transaction sender in this context is defined as `msg.sender` in Solidity semantics.
+The contract MUST support transactions sent from externally-owned accounts (EOA).
+The contract MAY also support meta-transactions sent via paymasters or relayers,
+which MAY require additional authentication-related logic.
+
 The holder and the keeper MAY be different entities for the same membership.
 When authorizing membership-related requests,
 the contract SHOULD distinguish between the keeper and non-keepers,