iommu: Fix passthrough option documentation

Document that the default for "iommu.passthrough" is now configurable.

Fixes: 58d11317 ("iommu: Add config option to set passthrough as default")
Signed-off-by: NRobin Murphy <robin.murphy@arm.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -1754,7 +1754,7 @@
 			Format: { "0" | "1" }
 			0 - Use IOMMU translation for DMA.
 			1 - Bypass the IOMMU for DMA.
-			unset - Use IOMMU translation for DMA.
+			unset - Use value of CONFIG_IOMMU_DEFAULT_PASSTHROUGH.
 
 	io7=		[HW] IO7 for Marvel based alpha systems
 			See comment before marvel_specify_io7 in