qemu-options.hx: Document the arm-smmuv3 device

Now that arm,virt can have user-creatable smmuv3 devices, document it.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Signed-off-by: Shameer Kolothum <skolothumtho@nvidia.com>
Reviewed-by: Donald Dutile <ddutile@redhat.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Message-id: 20250829082543.7680-9-skolothumtho@nvidia.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

qemu-options.hx

@@ -1231,6 +1231,13 @@ SRST
     ``aw-bits=val`` (val between 32 and 64, default depends on machine)
         This decides the address width of the IOVA address space.
 
+``-device arm-smmuv3,primary-bus=id``
+    This is only supported by ``-machine virt`` (ARM).
+
+    ``primary-bus=id``
+        Accepts either the default root complex (pcie.0) or a
+        pxb-pcie based root complex.
+
 ERST
 
 DEF("name", HAS_ARG, QEMU_OPTION_name,