Documentation
Author: LysandreJik
Pull request: yes

Updating documentation for tokenizers.

~Still left to do:~ will do in a future PR

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2989?src=pr&el=h1) Report
> Merging [#2989](https://codecov.io/gh/huggingface/transformers/pull/2989?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c913eb9c3894b4031dc059d22b42e38a5fcef989?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2989/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2989?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2989      +/-   ##
==========================================
+ Coverage   77.26%   77.27%   +<.01%     
==========================================
  Files          98       98              
  Lines       16040    16047       +7     
==========================================
+ Hits        12393    12400       +7     
  Misses       3647     3647
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2989?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/2989/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3hsbS5weQ==) | `96.22% <ø> (ø)` | :arrow_up: |
| [src/transformers/configuration\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/2989/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2dwdDIucHk=) | `97.05% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2989/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `88.16% <ø> (ø)` | :arrow_up: |
| [src/transformers/tokenization\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/2989/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fY3RybC5weQ==) | `96.19% <ø> (ø)` | :arrow_up: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/2989/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `96.05% <ø> (ø)` | :arrow_up: |
| [src/transformers/tokenization\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/2989/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZGlzdGlsYmVydC5weQ==) | `100% <ø> (ø)` | :arrow_up: |
| [src/transformers/tokenization\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/2989/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZmxhdWJlcnQucHk=) | `40.42% <ø> (ø)` | :arrow_up: |
| [src/transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/2989/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fb3BlbmFpLnB5) | `82.5% <ø> (ø)` | :arrow_up: |
| [src/transformers/configuration\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/2989/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2ZsYXViZXJ0LnB5) | `100% <ø> (ø)` | :arrow_up: |
| [src/transformers/tokenization\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/2989/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fZ3B0Mi5weQ==) | `96.87% <ø> (ø)` | :arrow_up: |
| ... and [9 more](https://codecov.io/gh/huggingface/transformers/pull/2989/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2989?src=pr&el=footer). Last update [c913eb9...b393150](https://codecov.io/gh/huggingface/transformers/pull/2989?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).