[docs] fixed params description

[StrikerRUS]

Refer to #2163 (comment).

@guolinke To be honest, I'm not sure that pos_bagging_fraction and neg_bagging_fraction are used in MultiClassOVA. It seems to me that according to the code below, num_pos_data = 0 and balance_bagging_cond = false because MultiClassOVA objective doesn't override NumPositiveData().

LightGBM/src/boosting/gbdt.cpp

Lines 728 to 732 in 55656f2

	data_size_t num_pos_data = 0;
	if (objective_function_ != nullptr) {
	num_pos_data = objective_function_->NumPositiveData();
	}
	bool balance_bagging_cond = (config->pos_bagging_fraction < 1.0 || config->neg_bagging_fraction < 1.0) && (num_pos_data > 0);

Please tell how it is actually working.

[StrikerRUS]

@guolinke Was I right about *_bagging_fraction params?

[guolinke]

I think they are only used for binary classification.