chainrpc: clarify documentation

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

lnrpc/chainrpc/chainnotifier.swagger.json

@@ -62,7 +62,7 @@
     "/v2/chainnotifier/register/confirmations": {
       "post": {
         "summary": "RegisterConfirmationsNtfn is a synchronous response-streaming RPC that\nregisters an intent for a client to be notified once a confirmation request\nhas reached its required number of confirmations on-chain.",
-        "description": "A client can specify whether the confirmation request should be for a\nparticular transaction by its hash or for an output script by specifying a\nzero hash.",
+        "description": "A confirmation request must have a valid output script. It is also possible\nto give a transaction ID. If the transaction ID is not set, a notification\nis sent once the output script confirms. If the transaction ID is also set,\na notification is sent once the output script confirms in the given\ntransaction.",
         "operationId": "ChainNotifier_RegisterConfirmationsNtfn",
         "responses": {
           "200": {