doc: gorums.NewServer claims to be for internal use

[meling]

The documentation in server.go claims:

// NewServer returns a new instance of GorumsServer.
// This function is intended for internal Gorums use.
// You should call `NewServer` in the generated code instead.
func NewServer(opts ...ServerOption) *Server {

However, this function is not called by generated code but only by users. There is no NewServer in the generated code. We should remove the incorrect description.