Documentation: cpufreq: intel_pstate: enhance documentation

This is an attempt to make documentation more user friendly.
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Reviewed-by: Doug Smythies <dsmythies@telus.net>
Reviewed-by: Chen, Yu C <yu.c.chen@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>