routerrpc: update QueryProbability documentation

Changes the docstring of QueryProbability to reflect changes that were
introduced in #6857.

lnrpc/routerrpc/router.swagger.json

@@ -116,7 +116,7 @@
     },
     "/v2/router/mc/probability/{from_node}/{to_node}/{amt_msat}": {
       "get": {
-        "summary": "Deprecated. QueryProbability returns the current success probability\nestimate for a given node pair and amount.",
+        "summary": "Deprecated. QueryProbability returns the current success probability\nestimate for a given node pair and amount. The call returns a zero success\nprobability if no channel is available or if the amount violates min/max\nHTLC constraints.",
         "operationId": "Router_QueryProbability",
         "responses": {
           "200": {