drm/xe/hw_engine: document top-down paging requirement

We were doing this anyway, but going forward for paging engines,
agreement is to always reserve BCS instances in top down fashion. This
hopefully future proofs things for VFs, where in some low-level places
it might only have the physical BCS instance from the hw pov. If we
stick to a consistent mapping scheme, it should make it possible to
determine if this is a special paging engine, or not.

v2 (Daniele)
  - Give a concrete example, like with page fault descriptor

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patch.msgid.link/20260626111520.487997-21-matthew.auld@intel.com

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