Add note about doc generation
Just make it explicit that doc generation is only for local inspection.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4770?src=pr&el=h1) Report
> Merging [#4770](https://codecov.io/gh/huggingface/transformers/pull/4770?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e645b9ab9407e1c1b2c168317dc79fe13fc6e0b4&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4770/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4770?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4770      +/-   ##
==========================================
- Coverage   77.31%   77.26%   -0.05%     
==========================================
  Files         128      128              
  Lines       21059    21059              
==========================================
- Hits        16281    16271      -10     
- Misses       4778     4788      +10     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4770?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/4770/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdHJhbnNmb194bC5weQ==) | `36.80% <0.00%> (-3.88%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4770/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.79% <0.00%> (+0.16%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4770/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `74.27% <0.00%> (+0.41%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/4770/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `82.03% <0.00%> (+1.35%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4770?src=pr&el=footer). Last update [e645b9a...39dc245](https://codecov.io/gh/huggingface/transformers/pull/4770?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
