drm/amdgpu/ras: document the reboot ras option

We recently added it, but never documented it.
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c

@@ -215,11 +215,12 @@ static struct ras_manager *amdgpu_ras_find_obj(struct amdgpu_device *adev,
  * value to the address.
  *
  * Second member: struct ras_debug_if::op.
- * It has three kinds of operations.
+ * It has four kinds of operations.
  *
  * - 0: disable RAS on the block. Take ::head as its data.
  * - 1: enable RAS on the block. Take ::head as its data.
  * - 2: inject errors on the block. Take ::inject as its data.
+ * - 3: reboot on unrecoverable error
  *
  * How to use the interface?
  * programs: