Document changes in behavior against i915

We already have one change in behavior against i915 and another change in behavior under discussion. All of changes in behavior needs to be documented so all UMDs are aware of it.

• The MI_BATCH_BUFFER_START third level chained batch buffer is disabled for all platforms on i915 while enabled in gfx12.5+(default HW state in gfx12.5+) in Xe, see discussion in this thread: https://lists.freedesktop.org/archives/intel-xe/2023-January/000100.html