add documentation for the systemd nest feature for XRDP

admin-guide/configuration/xrdp.md

@@ -28,3 +28,9 @@ Also you may choose the backend for the virtual sessions, either `libxup.so` (de
 ```
 xrdp::virtual_desktop::backend: 'libvnc.so'
 ```
+
+If you want to allow the same user to be able to open in parallel a local and a remote session then you can enable the systemd nest feature:
+
+```
+xrdp::nest_systemd::enable: true
+```