Fix docstring for `BertTokenizerFast`.
Author: PhilipMay
Pull request: yes

- remove duplicate doc-entry for `tokenize_chinese_chars`
- add doc for `strip_accents` and `wordpieces_prefix`

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6185?src=pr&el=h1) Report
> Merging [#6185](https://codecov.io/gh/huggingface/transformers/pull/6185?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a39dfe4fb122c11be98a563fb8ca43b322e01036&el=desc) will **increase** coverage by `1.25%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6185/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6185?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6185      +/-   ##
==========================================
+ Coverage   78.34%   79.59%   +1.25%     
==========================================
  Files         146      146              
  Lines       26607    26607              
==========================================
+ Hits        20844    21178     +334     
+ Misses       5763     5429     -334     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6185?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6185/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmVydC5weQ==) | `91.32% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6185/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `24.22% <0.00%> (-63.98%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6185/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9iZXJ0LnB5) | `70.32% <0.00%> (-26.66%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6185/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `80.30% <0.00%> (+0.25%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/6185/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `98.79% <0.00%> (+34.61%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6185/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `96.77% <0.00%> (+73.38%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6185?src=pr&el=footer). Last update [a39dfe4...f44324f](https://codecov.io/gh/huggingface/transformers/pull/6185?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).