chainrpc: clarify documentation

Clarify the comment on RegisterConfirmationsNtfn. The output script
is in fact mandatory, not optional.

lnrpc/chainrpc/chainnotifier.proto

@@ -12,9 +12,11 @@ service ChainNotifier {
     registers an intent for a client to be notified once a confirmation request
     has reached its required number of confirmations on-chain.
 
-    A client can specify whether the confirmation request should be for a
-    particular transaction by its hash or for an output script by specifying a
-    zero hash.
+    A confirmation request must have a valid output script. It is also possible
+    to give a transaction ID. If the transaction ID is not set, a notification
+    is sent once the output script confirms. If the transaction ID is also set,
+    a notification is sent once the output script confirms in the given
+    transaction.
     */
     rpc RegisterConfirmationsNtfn (ConfRequest) returns (stream ConfEvent);