cpuidle: menu: Update documentation after get_typical_interval() changes

The documentation of the menu cpuidle governor needs to be updated
to match the code behavior after some changes made recently.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Link: https://patch.msgid.link/4998484.31r3eYUQgx@rjwysocki.net


[ rjw: More specific subject, two typos fixed in the changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>