doc: add intel_pstate in power management

doc: add intel_pstate in power management

With the addition of the patchset to allow auto-detection and use
of the intel_pstate kernel driver instead of the acpi-cpufreq kernel
driver, we need to reflect this in the documentation.

Now, instead of telling the user to disable intel_pstate in all cases,
it is now an optional step, should the user want to use acpi-cpufreq.

Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: Marko Kovacevic <marko.kovacevic@intel.com>