cpufreq: docs: userspace: Explain HW coordination influence

Extend the "scaling_setspeed" sysfs attribute description in the
userspace governor documentation to cover possible differences
between the requested and actual CPU frequency.

Signed-off-by: Shashank Balaji <shashank.mahadasyam@sony.com>
Link: https://patch.msgid.link/20250527-userspace-governor-doc-v2-2-0e22c69920f2@sony.com


[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>