[docs] added FAQ on early stopping (fixes #2270) (#2272)

* added FAQ on early stopping (fixes #2270)

* fixed indentation in FAQ

* Update docs/FAQ.rst
Co-Authored-By: Nikita Titov <nekit94-08@mail.ru>

docs/FAQ.rst

@@ -167,6 +167,16 @@ LightGBM
 
 --------------
 
+-  **Question 12**: Why is early stopping not enabled by default in LightGBM?
+
+-  **Solution 12**: Early stopping involves choosing a validation set, a special type of holdout which is used to evaluate the current state of the model after each iteration to see if training can stop.
+
+   In ``LightGBM``, `we have decided to require that users specify this set directly <./Parameters.rst#valid>`_. Many options exist for splitting training data into training, test, and validation sets.
+
+   The appropriate splitting strategy depends on the task and domain of the data, information that a modeler has but which ``LightGBM`` as a general-purpose tool does not.
+
+--------------
+
 R-package
 ~~~~~~~~~
 

docs/README.rst

@@ -20,8 +20,7 @@ You can build the documentation locally. Just install Doxygen and run in ``docs`
     pip install -r requirements.txt
     make html
 
-If you faced any problems with Doxygen installation or you simply do not need documentation for C code,
-it is possible to build the documentation without it:
+If you faced any problems with Doxygen installation or you simply do not need documentation for C code, it is possible to build the documentation without it:
 
 .. code:: sh