[Tokenizer Doc] Improve tokenizer summary
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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This PR improves the tokenizer docs. This includes more consistent use of terminology, arguably better phrasing, correction of spelling, and more consistent formatting. 

Terminology: Tried to make the difference between "symbol", "character", "word", and "subword"

Consistency: Use `"` for token notation and replace `this paper <...>`__ by `<paper_name (author, year)>`, rename section to "Summary of the tokenizers"

I want to spend some time on the tokenizers library in Rust in the next couple of weeks and was reading through this summary for a start. It's great! I thought that I can improve the wording and explanations a bit while reading through it and pass it through Grammarly.


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