doc: explain rt list exportation with route-maps

Explain that an export route target list can be configured
alternatively by using route-maps.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>

doc/user/bgp.rst

@@ -2996,7 +2996,9 @@ address-family:
 
    Specifies the route-target list to be attached to a route (export) or the
    route-target list to match against (import) when exporting/importing between
-   the current unicast VRF and VPN.
+   the current unicast VRF and VPN. The `rt vpn export RTLIST` command is not
+   mandatory and can be replaced or completed by the `set extcommunity rt`
+   command in the route-map attached with the `route-map vpn export`.
 
    The RTLIST is a space-separated list of route-targets, which are BGP
    extended community values as described in