drm/amdgpu/mn: fix documentation for amdgpu_mn_read_lock

Document the new parameter.

Fixes: 93065ac7 ("mm, oom: distinguish blockable mode for mmu notifiers")
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>