iommu: Add a kdoc to iommu_unmap()

Describe the most conservative version of the driver implementations.
All drivers should support this.

Many drivers support extending the range if a large page is hit, but
let's not make that officially approved API. The main point is to
document explicitly that split is not supported.

Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/3-v3-b3a5b5937f56+7bb-arm_no_split_jgg@nvidia.com


Signed-off-by: Will Deacon <will@kernel.org>