ModelTrainer supports SDK Defaults Config but is not in the Documentations

**What did you find confusing? Please describe.**
* https://sagemaker.readthedocs.io/en/stable/overview.html#configuring-and-using-defaults-with-the-sagemaker-python-sdk
* ModelTrainer has support but not in the `readthedocs` - https://github.com/aws/sagemaker-python-sdk/blob/master/src/sagemaker/modules/train/model_trainer.py#L278

**Describe how documentation can be improved**
A clear and concise description of where documentation was lacking and how it can be improved.

**Additional context**
Add any other context or screenshots about the documentation request here.