Fix docs for early stopping (#865)

* updated docs

* updated docs

* upd

docs/source/early_stopping.rst

@@ -1,16 +1,22 @@
 Early stopping
-==================
+==============
 
+Default behavior
+----------------
+By default early stopping will be enabled if `'val_loss'`
+is found in `validation_end()` return dict. Otherwise
+training will proceed with early stopping disabled.
 
 Enable Early Stopping
-----------------------
+---------------------
 There are two ways to enable early stopping.
 
 .. note:: See: :ref:`trainer`
 
 .. code-block:: python
 
-    # A) Looks for val_loss in validation_step return dict
+    # A) Set early_stop_callback to True. Will look for 'val_loss'
+    # in validation_end() return dict. If it is not found an error is raised.
     trainer = Trainer(early_stop_callback=True)
 
     # B) Or configure your own callback
@@ -23,13 +29,10 @@ There are two ways to enable early stopping.
     )
     trainer = Trainer(early_stop_callback=early_stop_callback)
 
-Force disable early stop
--------------------------------------
-To disable early stopping pass None to the early_stop_callback
+Disable Early Stopping
+----------------------
+To disable early stopping pass ``False`` to the `early_stop_callback`.
+Note that ``None`` will not disable early stopping but will lead to the
+default behaviour.
 
 .. note:: See: :ref:`trainer`
-
-.. code-block:: python
-
-   # DEFAULT
-   trainer = Trainer(early_stop_callback=None)