cpuidle: menu: Update documentation after previous changes

After commit 38f83090 ("cpuidle: menu: Remove iowait influence") and
other previous changes, the description of the menu governor in the
documentation does not match the code any more, so update it as
appropriate.

Fixes: 38f83090 ("cpuidle: menu: Remove iowait influence")
Fixes: 5484e31b ("cpuidle: menu: Skip tick_nohz_get_sleep_length() call in some cases")
Reported-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Link: https://patch.msgid.link/12589281.O9o76ZdvQC@rjwysocki.net