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 :-)