cpufreq: Add doc style comment about cpufreq_cpu_{get|put}()

This clearly states what the code inside these routines is doing and how these
must be used.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>