[docs] Quick fix for better understanding for forced split logic (#1784)

* quick fix for better understanding

* update document for forced split

* typo fix

* made NOTE bold

* made Note bold

docs/Parameters.rst

@@ -362,6 +362,8 @@ Learning Control Parameters
 
    -  categorical splits are forced in a one-hot fashion, with ``left`` representing the split containing the feature value and ``right`` representing other values
 
+   -  **Note**: the forced split logic will be ignored, if the split makes gain worse
+
    -  see `this file <https://github.com/Microsoft/LightGBM/tree/master/examples/binary_classification/forced_splits.json>`__ as an example
 
 -  ``refit_decay_rate`` :raw-html:`<a id="refit_decay_rate" title="Permalink to this parameter" href="#refit_decay_rate">&#x1F517;&#xFE0E;</a>`, default = ``0.9``, type = double, constraints: ``0.0 <= refit_decay_rate <= 1.0``

include/LightGBM/config.h

@@ -365,6 +365,7 @@ public:
   // desc = path to a ``.json`` file that specifies splits to force at the top of every decision tree before best-first learning commences
   // desc = ``.json`` file can be arbitrarily nested, and each split contains ``feature``, ``threshold`` fields, as well as ``left`` and ``right`` fields representing subsplits
   // desc = categorical splits are forced in a one-hot fashion, with ``left`` representing the split containing the feature value and ``right`` representing other values
+  // desc = **Note**: the forced split logic will be ignored, if the split makes gain worse
   // desc = see `this file <https://github.com/Microsoft/LightGBM/tree/master/examples/binary_classification/forced_splits.json>`__ as an example
   std::string forcedsplits_filename = "";
 

src/treelearner/feature_histogram.hpp

@@ -336,7 +336,7 @@ public:
     // gain with split is worse than without split
     if (std::isnan(current_gain) || current_gain <= min_gain_shift) {
       output->gain = kMinScore;
-      Log::Warning("Gain with forced split worse than without split");
+      Log::Warning("'Forced Split' will be ignored since the gain getting worse. ");
       return;
     };
 
@@ -393,7 +393,7 @@ public:
                            meta_->config->max_delta_step);
     if (std::isnan(current_gain) || current_gain <= min_gain_shift) {
       output->gain = kMinScore;
-      Log::Warning("Gain with forced split worse than without split");
+      Log::Warning("'Forced Split' will be ignored since the gain getting worse. ");
       return;
     }