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.proto

@@ -124,6 +124,10 @@ service Router {
     BuildRoute builds a fully specified route based on a list of hop public
     keys. It retrieves the relevant channel policies from the graph in order to
     calculate the correct fees and time locks.
+    Note that LND will use its default final_cltv_delta if no value is supplied.
+    Make sure to add the correct final_cltv_delta depending on the invoice
+    restriction. Moreover the caller has to make sure to provide the
+    payment_addr if the route is paying an invoice which signaled it.
     */
     rpc BuildRoute (BuildRouteRequest) returns (BuildRouteResponse);