Enable Nested fiedsets to be in Lists

With this changes you can put a OptionalNestedField in a List field by
peserving all the features of nested Fieldsets.

Signed-off-by: Jan Losinski <losinski@wh2.tu-dresden.de>
Add Unittests for Nested fieldsets in lists.

Signed-off-by: Jan Losinski <losinski@wh2.tu-dresden.de>
add documentation to ObjectMemberField

Signed-off-by: Jan Losinski <losinski@wh2.tu-dresden.de>
Add documntation to Fieldset

Signed-off-by: Jan Losinski <losinski@wh2.tu-dresden.de>
Add documentation to OptionalNestedField

Signed-off-by: Jan Losinski <losinski@wh2.tu-dresden.de>
Change documentation hint

Signed-off-by: Jan Losinski <losinski@wh2.tu-dresden.de>