cpufreq: docs: Add missing cpuinfo_cur_freq description

Add a description of the cpuinfo_cur_freq policy attribute in sysfs
to the cpufreq documentation under Documentation/admin-guide/pm/ as
it is missing after commit 2a0e4927 (cpufreq: User/admin
documentation update and consolidation) that overlooked it.

Fixes: 2a0e4927

 (cpufreq: User/admin documentation update and consolidation)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>