drm/amdgpu: Add documentation for enforce isolation feature

drm/amdgpu: Add documentation for enforce isolation feature

This feature enables process isolation on the graphics engine by
serializing access to it and adding a cleaner shader which clears LDS
(Local Data Store) and GPRs (General Purpose Registers) between jobs.

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Suggested-by: Asad Kamal <asad.kamal@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>