routerrpc: use cltv delta default for buildroute.

This commit uses the default timelock delta (cltv delta) when
no value is provided for the buildroute rpc cmd.
Moreover it enhances the docs.

lnrpc/routerrpc/router.swagger.json

@@ -293,7 +293,7 @@
     },
     "/v2/router/route": {
       "post": {
-        "summary": "lncli: `buildroute`\nBuildRoute builds a fully specified route based on a list of hop public\nkeys. It retrieves the relevant channel policies from the graph in order to\ncalculate the correct fees and time locks.",
+        "summary": "lncli: `buildroute`\nBuildRoute builds a fully specified route based on a list of hop public\nkeys. It retrieves the relevant channel policies from the graph in order to\ncalculate the correct fees and time locks.\nNote that LND will use its default final_cltv_delta if no value is supplied.\nMake sure to add the correct final_cltv_delta depending on the invoice\nrestriction. Moreover the caller has to make sure to provide the\npayment_addr if the route is paying an invoice which signaled it.",
         "operationId": "Router_BuildRoute",
         "responses": {
           "200": {