Update documentation about the deprecation of train_cfg/test_cfg (#4489)

* Update documentation about the modification of train_cfg/test_cfg

* Fix docs str

* Add docs str

* Add docs str

docs/tutorials/config.md

@@ -64,6 +64,32 @@ We follow the below style to name config files. Contributors are advised to foll
     For `20e`, initial learning rate decays by a factor of 10 at the 16th and 19th epochs.
 - `{dataset}`: dataset like `coco`, `cityscapes`, `voc_0712`, `wider_face`.
 
+## Deprecated train_cfg/test_cfg
+
+The `train_cfg` and `test_cfg` are deprecated in config file, please specify them in the model config. The original config structure is as below.
+
+```python
+# deprecated
+model = dict(
+   type=...,
+   ...
+)
+train_cfg=dict(...)
+test_cfg=dict(...)
+```
+
+The migration example is as below.
+
+```python
+# recommended
+model = dict(
+   type=...,
+   ...
+   train_cfg=dict(...),
+   test_cfg=dict(...),
+)
+```
+
 ## An Example of Mask R-CNN
 
 To help the users have a basic idea of a complete config and the modules in a modern detection system,
@@ -157,7 +183,7 @@ model = dict(
                 type='CrossEntropyLoss',  # Type of loss used for segmentation
                 use_mask=True,  # Whether to only train the mask in the correct class.
                 loss_weight=1.0))))  # Loss weight of mask branch.
-train_cfg = dict(  # Config of training hyperparameters for rpn and rcnn
+    train_cfg = dict(  # Config of training hyperparameters for rpn and rcnn
         rpn=dict(  # Training config of rpn
             assigner=dict(  # Config of assigner
                 type='MaxIoUAssigner',  # Type of assigner, MaxIoUAssigner is used for many common detectors. Refer to https://github.com/open-mmlab/mmdetection/blob/master/mmdet/core/bbox/assigners/max_iou_assigner.py#L10 for more details.
@@ -200,7 +226,7 @@ train_cfg = dict(  # Config of training hyperparameters for rpn and rcnn
             mask_size=28,  # Size of mask
             pos_weight=-1,  # The weight of positive samples during training.
             debug=False))  # Whether to set the debug mode
-test_cfg = dict(  # Config for testing hyperparameters for rpn and rcnn
+    test_cfg = dict(  # Config for testing hyperparameters for rpn and rcnn
         rpn=dict(  # The config to generate proposals during testing
             nms_across_levels=False,  # Whether to do NMS for boxes across levels
             nms_pre=1000,  # The number of boxes before NMS