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: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>