Update doc of the model page
Author: sgugger
Pull request: yes

Fixes docstring to conform to sphinx syntax and rephrase when needed. Also fixed bad copy-pastes from PyTorch to TF.

Preview is [here](https://64023-155220641-gh.circle-artifacts.com/0/docs/_build/html/main_classes/model.html).

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5985?src=pr&el=h1) Report
> Merging [#5985](https://codecov.io/gh/huggingface/transformers/pull/5985?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c3206eef44e9fbfca9ed4527f528107fcba31888&el=desc) will **decrease** coverage by `0.39%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5985/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5985?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5985      +/-   ##
==========================================
- Coverage   78.65%   78.26%   -0.40%     
==========================================
  Files         146      146              
  Lines       26227    26230       +3     
==========================================
- Hits        20630    20530     -100     
- Misses       5597     5700     +103     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5985?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5985/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.96% <100.00%> (+0.07%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5985/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `87.17% <100.00%> (+0.02%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5985/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `20.27% <0.00%> (-74.92%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/5985/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.02% <0.00%> (-1.29%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5985/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.96% <0.00%> (-0.26%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5985/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `81.49% <0.00%> (+0.29%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/5985/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9yb2JlcnRhLnB5) | `93.36% <0.00%> (+49.37%)` | :arrow_up: |

------

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> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5985?src=pr&el=footer). Last update [c3206ee...bfd8d3a](https://codecov.io/gh/huggingface/transformers/pull/5985?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).