Documentation: intel_pstate: Document HWP energy/performance hints

Updated documentation for the support of energy performance hint in
the HWP mode.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
[ rjw: Subject & changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>