cpufreq: Update scaling_cur_freq documentation

Commit f8475cef "x86: use common aperfmperf_khz_on_cpu() to
calculate KHz using APERF/MPERF" modified the way the scaling_cur_freq
cpufreq policy attribute in sysfs is handled on contemporary
Intel-based x86 systems, so update the documentation to reflect
that change.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>