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_server.go

@@ -999,6 +999,12 @@ func (s *Server) BuildRoute(ctx context.Context,
 		payAddr = &backingPayAddr
 	}
 
+	if req.FinalCltvDelta == 0 {
+		req.FinalCltvDelta = int32(
+			s.cfg.RouterBackend.DefaultFinalCltvDelta,
+		)
+	}
+
 	// Build the route and return it to the caller.
 	route, err := s.cfg.Router.BuildRoute(
 		amt, hops, outgoingChan, req.FinalCltvDelta, payAddr,