docs: cgroup-v1: Clarify that domain levels are system-specific

Add a clarification that domain levels are system-specific
and where to check for system details.

Signed-off-by: Vitalii Bursov <vitaly@bursov.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/42b177a2e897cdf880caf9c2025f5b609e820334.1714488502.git.vitaly@bursov.com