Fix `LayoutXLM` docstrings
Author: qqaatw
Pull request: yes

# What does this PR do?

A follow-up PR for #16187.

Also fix a legacy issue that the `ENCODE_PLUS_ADDITIONAL_KWARGS_DOCSTRING` of both `layoutlmv2` and `layoutxlm` is incorrect, which should be like [this](https://github.com/huggingface/transformers/blob/ff846e9b28358e5741dea5058433f7bcf8e7de76/src/transformers/tokenization_utils_base.py#L1311-L1363) instead of a copy of `ENCODE_KWARGS_DOCSTRING`.
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Fixes # (issue)

@NielsRogge @LysandreJik 

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- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Comments:
_The documentation is not available anymore as the PR was closed or 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.

ok got closed without a reason.

Hi @qqaatw,

Apologies this didn't get merged yet. I would like to re-open the PR, but seems like that's not possible anymore as the branch is deleted. Could you open a new PR?

Apologies again for how this was treated.

@NielsRogge I restored the branch, I don't know why it was deleted on my behalf.

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.

Hi @qqaatw,

feel free to re-open this PR and apply the suggestions

@NielsRogge I'm not able to re-open it. There is no button for re-opening.