message: Improve help message in num_threads
diff: @@ -10,16 +10,18 @@ class arg_num_threads : public int_argument {
  public:
  arg_num_threads() : int_argument() {
  _name = "num_threads";
  - _description = std::string(
  - "Number of threads available to the program. For full effect, the "
  - "model must be compiled with STAN_THREADS=true.");
  #ifdef STAN_THREADS
  + _description = std::string("Number of threads available to the program.");
  _validity = "num_threads > 0 || num_threads == -1";
  + _default
  + = "1 or the value of the STAN_NUM_THREADS environment variable if set.";
  #else
  + _description = std::string(
  + "Number of threads available to the program. To use this "
  + "argument, re-compile this model with STAN_THREADS=true.");
  _validity = "num_threads == 1";
  + _default = "1";
  #endif
  - _default
  - = "1 or the value of the STAN_NUM_THREADS environment variable if set.";
  _default_value = stan::math::internal::get_num_threads();
  _good_value = 1.0;
  _bad_value = -2.0;
  