Fix `LayoutLMv2` tokenization docstrings
Author: qqaatw
Pull request: yes

# What does this PR do?

The tokenizers of `LayoutLMv2` don't accept `is_split_into_words` keyword for `__call__`. Thus, I created a dedicated kwargs docstring to replace the one in `src/transformers/tokenization_utils_base.py` being used.

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Fixes # (issue)


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Will open a follow-up PR for `LayoutXLM` after accepted and merged.

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.

@NielsRogge can you give this a look when you have a minute?

Done.

Seems like you changed some files which shouldn't be changed, like `index.mdx`. Could you fix those, please?

Thanks, merging!