Update documentation regarding how to access and change dataset keys

[SarahL88]

A tester (Isabel) noted that it was difficult to remember how to access/change dataset keys. Please update the documentation accordingly.

Tester's comment:

"One other convenience thing -- I don't think I've seen a function that easily changes the dataset dictionary keys (like token_ids)? Currently I'm just doing this through a small custom data collator. But it might be nice to have a dataset method that can just change the keys of the dataset

"On second thought, I'm not sure that this needs to be a method. But I think it would be really useful to mention it in the documentation! It took me a bit of time to remember that the way to access and change dataset keys is through a custom data collator, and how to write one."