add custom_objects docstring (#1602)
add custom_objects docstring (#1602)

* add custom_objects docstring

* nits on docs

* responses to comments on PR

* grammatical fix

* Minor tweaks

* Phrasing fix

mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow