[finetune trainer] better logging and help
As a follow up to this [thread](https://discuss.huggingface.co/t/summarization-is-finetune-trainer-py-accepting-length-arguments-correctly/2879/) this PR:

* documents that `--val_max_target_length` is also used during `generate`
* disambiguates `use_task_specific_params` logger so that it's clear that it dumps just the initial params and that those could be overridden by user's cl args

@sgugger 