cpufreq: Improve kerneldoc comments for cpufreq_cpu_get/put()

Fix the formatting of the cpufreq_cpu_get() and cpufreq_cpu_put()
kerneldoc comments and rework them to be somewhat easier to follow.

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