Example of pad_to_multiple_of for padding and truncation guide & docstring update
This PR adds a minor update to the docs as previously it was not clear that `pad_to_multiple_of` has to be used with `padding=True`.
Based on https://huggingface.slack.com/archives/C027NLU6CE9/p1679325764920509

_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for iterating!