docs: pcie: Spell out machine type needs for PCIe features

PCIe features are available only via the 'q35' machine type for x86 and
the 'virt' machine type for AArch64 architecture.

Mention that explicitly.

Thanks: Daniel Berrangé

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/pcie.txt

@@ -3,8 +3,13 @@ PCI EXPRESS GUIDELINES
 
 1. Introduction
 ================
-The doc proposes best practices on how to use PCI Express/PCI device
-in PCI Express based machines and explains the reasoning behind them.
+The doc proposes best practices on how to use PCI Express (PCIe) / PCI
+devices in PCI Express based machines and explains the reasoning behind
+them.
+
+Note that the PCIe features are available only when using the 'q35'
+machine type on x86 architecture and the 'virt' machine type on AArch64.
+Other machine types do not use PCIe at this time.
 
 The following presentations accompany this document:
  (1) Q35 overview.