pci: doc: Add comment about DSM_PCI_PRESERVE_BOOT_CONFIG in AML

The DSM_PCI_PRESERVE_BOOT_CONFIG function in the DSM tells the guest if
it should preserve the PCI resource assignments done by the firmware
(VMM in this case). Since we do not support BAR relocation this is a
valid way to prevent guest from trying to relocate them.

Signed-off-by: Egor Lazarchuk <yegorlz@amazon.co.uk>

Author: ShadowCurse

src/vmm/src/devices/pci/pci_segment.rs

@@ -316,6 +316,10 @@ impl Aml for PciDsmMethod {
                             &aml::Equal::new(&aml::Arg(2), &aml::ZERO),
                             vec![&aml::Return::new(&aml::Buffer::new(vec![0x21]))],
                         ),
+                        // Define DSM_PCI_PRESERVE_BOOT_CONFIG(0x05) function. The return
+                        // value of 0 indicates that the guest should preserve PCI resource
+                        // assignment done by us.
+                        // https://elixir.bootlin.com/linux/v6.19.8/source/drivers/pci/pci-acpi.c#L123
                         &aml::If::new(
                             &aml::Equal::new(&aml::Arg(2), &0x05u8),
                             vec![&aml::Return::new(&aml::ZERO)],