Outdated documentation for MultipleSelectionFormField

[IceQ1337]

The documentation (WSC 5.3 onwards) for MultipleSelectionFormField states that it implements the INullableFormField interface, but this is not the case since https://github.com/WoltLab/WCF/commit/0bb62eb9456527afbcc6a66b35bbe89036f919cc. This makes the following sentence of the documentation obsolete, since the field is not nullable and the save value must be explicitly set to null in e.g. ObjectAction::create() and ObjectAction::update().

"If the field is nullable and no option is selected, null is returned as the save value."