IOMMU: complete/correct comment explaining "iommu=" sub-options

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/passthrough/iommu.c
+++ b/xen/drivers/passthrough/iommu.c
@@ -31,13 +31,19 @@ static void iommu_dump_p2m_table(unsigne
  *
  *   off|no|false|disable       Disable IOMMU (default)
  *   force|required             Don't boot unless IOMMU is enabled
+ *   no-intremap                Disable interrupt remapping
+ *   verbose                    Be more verbose
+ *   debug                      Enable debugging messages and checks
  *   workaround_bios_bug        Workaround some bios issue to still enable
-                                VT-d, don't guarantee security
+ *                              VT-d, don't guarantee security
  *   dom0-passthrough           No DMA translation at all for Dom0
  *   dom0-strict                No 1:1 memory mapping for Dom0
+ *   no-sharept                 Don't share VT-d and EPT page tables
  *   no-snoop                   Disable VT-d Snoop Control
  *   no-qinval                  Disable VT-d Queued Invalidation
- *   no-intremap                Disable VT-d Interrupt Remapping
+ *   no-igfx                    Disable VT-d for IGD devices (insecure)
+ *   no-amd-iommu-perdev-intremap Don't use per-device interrupt remapping
+ *                              tables (insecure)
  */
 custom_param("iommu", parse_iommu_param);
 bool_t __initdata iommu_enable = 1;