Documentation: intel_pstate: PID tuning is not always available

PID tuning is not available when the get_target_pstate_use_cpu_load()
is used to calculate target_state. So update the documentation.
Signed-off-by: NSrinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/cpu-freq/intel-pstate.txt

@@ -122,11 +122,12 @@ will run at is selected by the processor itself.
 
 Tuning Intel P-State driver
 
-When HWP mode is not used, debugfs files have also been added to allow the
-tuning of the internal governor algorithm. These files are located at
-/sys/kernel/debug/pstate_snb/. The algorithm uses a PID (Proportional
-Integral Derivative) controller. The PID tunable parameters are:
+When the performance can be tuned using PID (Proportional Integral
+Derivative) controller, debugfs files are provided for adjusting performance.
+They are presented under:
+/sys/kernel/debug/pstate_snb/
 
+The PID tunable parameters are:
       deadband
       d_gain_pct
       i_gain_pct