minor doc fix (paritytech#2207)

bridges/primitives/chain-bridge-hub-cumulus/src/lib.rs

@@ -124,9 +124,13 @@ pub type Address = MultiAddress<AccountId, ()>;
 // `ensure_able_to_receive_confirmation` test.
 
 /// Maximal number of unrewarded relayer entries at inbound lane for Cumulus-based parachains.
+/// Note: this value is security-relevant, decreasing it should not be done without careful
+/// analysis (like the one above).
 pub const MAX_UNREWARDED_RELAYERS_IN_CONFIRMATION_TX: MessageNonce = 1024;
 
 /// Maximal number of unconfirmed messages at inbound lane for Cumulus-based parachains.
+/// Note: this value is security-relevant, decreasing it should not be done without careful
+/// analysis (like the one above).
 pub const MAX_UNCONFIRMED_MESSAGES_IN_CONFIRMATION_TX: MessageNonce = 4096;
 
 /// Extra signed extension data that is used by all bridge hubs.