[modeling_utils] better explanation of ignore keys
Integrating the improved explanation of ignore keys by @sgugger at https://github.com/huggingface/transformers/issues/16719#issuecomment-1096878395 with some tweaks from myself.

It's still unclear whether they should include the base model prefix or not, but we can sort it out when https://github.com/huggingface/transformers/issues/16719 gets more clarity

@sgugger 

_The documentation is not available anymore as the PR was closed or merged._