update docs to include warning about extension pack

builder/virtualbox/iso/builder.go

@@ -90,7 +90,8 @@ type Config struct {
 	// defaults to ide. When set to sata, the drive is attached to an AHCI SATA
 	// controller. When set to scsi, the drive is attached to an LsiLogic SCSI
 	// controller. When set to pcie, the drive is attached to an NVMe
-	// controller.
+	// controller. (Please note that when you use "pcie", you'll need to have
+	// Virtualbox 6 and an [extension pack](https://www.virtualbox.org/wiki/Downloads#VirtualBox6.0.14OracleVMVirtualBoxExtensionPack))
 	HardDriveInterface string `mapstructure:"hard_drive_interface" required:"false"`
 	// The number of ports available on any SATA controller created, defaults
 	// to 1. VirtualBox supports up to 30 ports on a maximum of 1 SATA

website/source/partials/builder/virtualbox/iso/_Config-not-required.html.md

@@ -48,7 +48,8 @@
     defaults to ide. When set to sata, the drive is attached to an AHCI SATA
     controller. When set to scsi, the drive is attached to an LsiLogic SCSI
     controller. When set to pcie, the drive is attached to an NVMe
-    controller.
+    controller. (Please note that when you use "pcie", you'll need to have
+    Virtualbox 6 and an [extension pack](https://www.virtualbox.org/wiki/Downloads#VirtualBox6.0.14OracleVMVirtualBoxExtensionPack))
     
 -   `sata_port_count` (int) - The number of ports available on any SATA controller created, defaults
     to 1. VirtualBox supports up to 30 ports on a maximum of 1 SATA