tools/power x86_energy_perf_policy.8: Document SoC Slider Options

x86_energy_perf_policy accesses the SoC Slider via standard
user/kernel APIs to the processor_thermal_soc_slider driver.

Machines that support SoC Slider largely use it instead of EPP,
which may continue to exist in a diminished role, or vanish entirely.

Signed-off-by: Len Brown <len.brown@intel.com>