Improved doxygen comments

middletier/cta/AdminHost.hpp

middletier/cta/AdminUser.hpp

middletier/cta/ArchivalJob.hpp

middletier/cta/EosRemoteStorage.hpp

middletier/cta/FileSystemNode.hpp

middletier/cta/FileSystemStorageClasses.hpp

middletier/cta/MiddleTierAdmin.hpp

middletier/cta/RemoteStorage.hpp

middletier/cta/RetrievalFileTransfer.hpp

middletier/cta/RetrievalJob.hpp

middletier/cta/SqliteMiddleTierAdmin.cpp

middletier/cta/SqliteMiddleTierAdmin.hpp

middletier/cta/SqliteMiddleTierUser.cpp

middletier/cta/SqliteMiddleTierUser.hpp

middletier/cta/Tape.hpp

middletier/cta/UserIdentity.hpp