[docs] clarify description of prediction early stopping (#4411)

docs/Parameters.rst

@@ -909,8 +909,12 @@ Predict Parameters
 
    -  used only in ``prediction`` task
 
+   -  used only in ``classification`` and ``ranking`` applications
+
    -  if ``true``, will use early-stopping to speed up the prediction. May affect the accuracy
 
+   -  **Note**: cannot be used with ``rf`` boosting type or custom objective function
+
 -  ``pred_early_stop_freq`` :raw-html:`<a id="pred_early_stop_freq" title="Permalink to this parameter" href="#pred_early_stop_freq">&#x1F517;&#xFE0E;</a>`, default = ``10``, type = int
 
    -  used only in ``prediction`` task

include/LightGBM/config.h

@@ -767,7 +767,9 @@ struct Config {
 
   // [no-save]
   // desc = used only in ``prediction`` task
+  // desc = used only in ``classification`` and ``ranking`` applications
   // desc = if ``true``, will use early-stopping to speed up the prediction. May affect the accuracy
+  // desc = **Note**: cannot be used with ``rf`` boosting type or custom objective function
   bool pred_early_stop = false;
 
   // [no-save]