Document librispeech configs
I think the main purpose of #4179 was how to be able to load both configs into one, so should we maybe add this part of the code: https://github.com/huggingface/datasets/issues/4179#issuecomment-1102383717 

to the doc? 

Actually @lhoestq would this work given that they have different split names: https://huggingface.co/datasets/librispeech_asr#data-splits ?  This doc extension does not explain why I can't simply load the whole dataset. Or what workaround I need to get the whole dataset, which is what people usually want for Librispeech. _The documentation is not available anymore as the PR was closed or merged._ @lhoestq, I can add a `"all"` config to Librispeech have the datasets already cached somewhere  I'm closing this PR then, feel free to continue the discussion in https://github.com/huggingface/datasets/issues/4179

Added an example of how to load one config or the other