docs: What does router bgp <AS> address-family l2vpn evpn vni <vni> route-target ... do?

The docs currently seem to lack an explicit description what

```
router bgp <AS>
   address-family l2vpn evpn
       vni <vni>
           route-target <import|export|both> ...
```

actually does.

It seems like this sets the MAC-VRF route target of the corresponding VNI, but I'd like to have this confirmed before making a PR to the docs.