cpufreq: docs: Add missing scaling_available_frequencies description

Add a description of the scaling_available_frequencies attribute in
sysfs to the documentation.
Signed-off-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://patch.msgid.link/20240701171040.369030-1-rgallaispou@gmail.comSigned-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/admin-guide/pm/cpufreq.rst

@@ -267,6 +267,10 @@ are the following:
 ``related_cpus``
 	List of all (online and offline) CPUs belonging to this policy.
 
+``scaling_available_frequencies``
+	List of available frequencies of the CPUs belonging to this policy
+	(in kHz).
+
 ``scaling_available_governors``
 	List of ``CPUFreq`` scaling governors present in the kernel that can
 	be attached to this policy or (if the |intel_pstate| scaling driver is