API Manual: FwOpen/FwClose are supported at class and instance level, in contrast to the manual

Firmware currently provides these two services at the class and instance levels of the Connecton Manager, and thus, the manual should conistently specify this. This shall be fixed.

On the other hand, according to the CIP specification, the services are required at instance level only, so we also have a minor deviation from the specification here. Anyway, since changing this one affects existing STC-Files, it should better be done with a new minor release.