docs: admin-guide: Add information about intel_pstate active mode

Information about intel_pstate active mode is added in the doc.
This operation mode could be used to set on the hardware when it's
not activated. Status of the mode could be checked from sysfs file
i.e., /sys/devices/system/cpu/intel_pstate/status.
The information is already available in cpu-freq/intel-pstate.txt
documentation.

Signed-off-by: Natesh Sharma <nsharma@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
[jc: reformatted for width ]
Link: https://lore.kernel.org/r/20230427083706.49882-1-nsharma@redhat.com