Update export format docstrings (#6151)

* Update export documentation

* Cleanup

* Update export.py

* Update README.md

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* Update README.md

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* Update train.py

* Update train.py

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train.py

@@ -1,10 +1,17 @@
 # YOLOv5 🚀 by Ultralytics, GPL-3.0 license
 """
-Train a YOLOv5 model on a custom dataset
+Train a YOLOv5 model on a custom dataset.
+
+Models and datasets download automatically from the latest YOLOv5 release.
+Models: https://github.com/ultralytics/yolov5/tree/master/models
+Datasets: https://github.com/ultralytics/yolov5/tree/master/data
+Tutorial: https://github.com/ultralytics/yolov5/wiki/Train-Custom-Data
 
 Usage:
-    $ python path/to/train.py --data coco128.yaml --weights yolov5s.pt --img 640
+    $ python path/to/train.py --data coco128.yaml --weights yolov5s.pt --img 640  # from pretrained (RECOMMENDED)
+    $ python path/to/train.py --data coco128.yaml --weights '' --cfg yolov5s.yaml --img 640  # from scratch
 """
+
 import argparse
 import math
 import os