This release does not contain the doxygen and neddoc documentations.
Both can be generated from source. After import of the project in the
OMNeT++ IDE select: "Project" -> "Generate NED documentation", select 
the project and hit OK. It may take some time to generate the
documentation.
You can also access the documentations online at:
http://core4inet.core-rg.de/signalsandgateways_documentation