taprpc: update AddInvoice docs

Since we now support setting the existing fields value/value_msat we
need to update the godoc in order to reflect the new behavior to the RPC
user.

Author: GeorgeTsagk

taprpc/tapchannelrpc/tapchannel.pb.go

@@ -179,13 +179,19 @@ message AddInvoiceRequest {
     // there are multiple channels with the given asset ID.
     bytes peer_pubkey = 3;
 
-    // The full lnd invoice request to send. All fields (except for the value
-    // and the route hints) behave the same way as they do for lnd's
-    // lnrpc.AddInvoice RPC method (see the API docs at
+    // The full lnd invoice request to send. All fields behave the same way as
+    // they do for lnd's lnrpc.AddInvoice RPC method (see the API docs at
     // https://lightning.engineering/api-docs/api/lnd/lightning/add-invoice
-    // for more details). The value/value_msat fields will be overwritten by the
-    // satoshi (or milli-satoshi) equivalent of the asset amount, after
-    // negotiating a quote with a peer that supports the given asset ID.
+    // for more details).
+    //
+    // Only one of the asset_amount/value/value_msat may be set to dictate the
+    // value of the invoice. When using asset_amount, the value/value_msat
+    // fields will be overwritten by the satoshi (or milli-satoshi) equivalent
+    // of the asset amount, after negotiating a quote with a peer that supports
+    // the given asset ID.
+    //
+    // If the value/value_msat are used, we still receive assets, but they will
+    // exactly evaluate to the defined amount in sats/msats.
     lnrpc.Invoice invoice_request = 4;
 
     // If set, then this will make the invoice created a hodl invoice, which

taprpc/tapchannelrpc/tapchannel.proto

@@ -1507,7 +1507,7 @@
         },
         "invoice_request": {
           "$ref": "#/definitions/lnrpcInvoice",
-          "description": "The full lnd invoice request to send. All fields (except for the value\nand the route hints) behave the same way as they do for lnd's\nlnrpc.AddInvoice RPC method (see the API docs at\nhttps://lightning.engineering/api-docs/api/lnd/lightning/add-invoice\nfor more details). The value/value_msat fields will be overwritten by the\nsatoshi (or milli-satoshi) equivalent of the asset amount, after\nnegotiating a quote with a peer that supports the given asset ID."
+          "description": "The full lnd invoice request to send. All fields behave the same way as\nthey do for lnd's lnrpc.AddInvoice RPC method (see the API docs at\nhttps://lightning.engineering/api-docs/api/lnd/lightning/add-invoice\nfor more details).\n\nOnly one of the asset_amount/value/value_msat may be set to dictate the\nvalue of the invoice. When using asset_amount, the value/value_msat\nfields will be overwritten by the satoshi (or milli-satoshi) equivalent\nof the asset amount, after negotiating a quote with a peer that supports\nthe given asset ID.\n\nIf the value/value_msat are used, we still receive assets, but they will\nexactly evaluate to the defined amount in sats/msats."
         },
         "hodl_invoice": {
           "$ref": "#/definitions/tapchannelrpcHodlInvoice",