[DOC] Add Usage Examples to Core Function Docstrings

[SimonBlanke]

Examples in docstrings provide various benefits for users and devs: You can see these examples during IDE hover tooltips, can generate examples for your auto generated documentation and can quickly see the usage pattern as a dev going through the code. Core public functions lack Examples sections in their docstrings.
Here are some of the public methods that would benefit from examples in docstrings:

• openml/datasets/functions.py:get_dataset()
• openml/datasets/functions.py:get_datasets()
• openml/tasks/functions.py:get_task()
• openml/runs/functions.py:run_model_on_task()
• openml/study/functions.py:get_suite()
• openml/study/functions.py:get_study()
• openml/study/functions.py:create_study()

[fkiraly]

I also noticed a few other issues with docstrings:

• missing defaults and types
• missing reference to retrieval from DB backend, e.g., what gets accessed
• missing reference to changes in database backend, e.g., what gets added, deleted