Better documentation for generation parameter defaults
# Generation default params documentation

It's very hard to follow how the generation parameters are set when running generation. When looking at the official function: https://github.com/huggingface/transformers/blob/700cee344691afc41f68aa18fedea463b22f95f1/src/transformers/generation_utils.py#L644 all parameters default to `None`, but are then later overwritten by the config's default parameters, *e.g.* here: https://github.com/huggingface/transformers/blob/700cee344691afc41f68aa18fedea463b22f95f1/src/transformers/generation_utils.py#L878 . This is very hard to trace or follow. We should at least put a warning or note that clearly states that all generation parameters (and actually all forward) parameters **always** default to the config.

What do you think @LysandreJik @patil-suraj @sgugger ? 

If you agree, I'll open a PR for it :-) 
Fine by me! I think it can just be stated at the beginning before each arg is documented.<|||||>Putting this is the "Improve generation task basket" so that this is handled once the generation docs are improved as well<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.