Documentation: use cases of coroutine by Learner and Runner not properly explained

[juandaanieel]

AsyncRunner checks if the function to be learned is a coroutine or not. This function comes from the learner, e.g. BaseLearner which says that the function might be modified by a subclass but gives no information about when or about it being a coroutine.

Documentation could be more explicit about:

• Learners accepting coroutines as functions. This is currently only mentioned in the runner's documentation.
• Mention in what cases the function is modified by the subclass of BaseLearner.