Clarify imagefolder is for small datasets
Based on feedback from [here](https://github.com/huggingface/datasets/issues/5317#issuecomment-1334108824), this PR adds a note to the `imagefolder` loading and creating docs that `imagefolder` is designed for small scale image datasets.
_The documentation is not available anymore as the PR was closed or merged._ I think it's also reasonable to add the same note to the AudioFolder decription Thank you ! I think "regular" is more appropriate than "small". It can easily scale to a few thousands of images - just not millions x) Replaced "small" with "several thousand" since what is considered "regular" and even "small" can be kind of vague!