For messages, a list of the content of the fields is
returned.
For message type descriptors, a list containing
nested type descriptors ('>Descriptor objects),
enum type descriptors ('>EnumDescriptor objects),
then field descriptors ('>FieldDescriptor objects) in that order.
For enum descriptors, a named list of the enumerated values.
For file descriptors, a named list of descriptors defined in the
specified file descriptor.
For service descriptors, ...