website/docs: document RDP raw args

website/docs/source/v2/cli/rdp.html.md

@@ -10,3 +10,17 @@ sidebar_current: "cli-rdp"
 This will start an RDP client for a remote desktop session with the
 guest. This only works for Vagrant environments that support remote
 desktop, which is typically only Windows.
+
+## Raw Arguments
+
+You can pass raw arguments through to your RDP client on the
+command-line by appending it after a `--`. Vagrant just passes
+these through. For example:
+
+```
+$ vagrant rdp -- /span
+...
+```
+
+The above command on Windows will execute `mstsc.exe /span config.rdp`,
+allowing your RDP to span multiple desktops.