intel_pstate: Update documentation of {max,min}_perf_pct sysfs files

Update documentation to make the interpretation of the values clearer

Link: https://bugzilla.kernel.org/show_bug.cgi?id=64251


Cc: 3.13+ <stable@vger.kernel.org>  # 3.13+
Signed-off-by: Dirk Brandewie <dirk.j.brandewie@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>