[RoBERTa] Minor clarifications to model doc
Author: bt2513
Pull request: yes

# What does this PR do?

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Hi there! When looking into the RoBERTa documentation, I noticed that some words were missing in the second point of the pre-training tricks (in addition to a typo "than" vs "that"). I thought it might be helpful to fix this and proposed minor suggestions to correct and clarify the documentation

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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@stevhliu @MKhalusova @sgugger 

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Models:

- text models: @ArthurZucker
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @zucchini-nlp (visual-language models) or @gante (all others)
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @muellerzr and @SunMarc

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- deepspeed: HF Trainer/Accelerate: @muellerzr
- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization (bitsandbytes, autogpt): @SunMarc 

Documentation: @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
- PyTorch: See Models above and tag the person corresponding to the modality of the example.
- TensorFlow: @Rocketknight1

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Comments:
Hi @stevhliu @MKhalusova @sgugger - I just wanted to follow up with this and get your thoughts on this PR. Should someone else be involved? Please let me know, thanks :)

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_31949). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.