[docstring] missing arg
add the missing `tie_word_embeddings` entry



# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6933?src=pr&el=h1) Report
> Merging [#6933](https://codecov.io/gh/huggingface/transformers/pull/6933?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e95d262f2553859af9bffbfe5f5bc7e362259939?el=desc) will **increase** coverage by `1.66%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6933/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6933?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6933      +/-   ##
==========================================
+ Coverage   77.70%   79.36%   +1.66%     
==========================================
  Files         161      161              
  Lines       30119    30119              
==========================================
+ Hits        23403    23905     +502     
+ Misses       6716     6214     -502     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6933?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/6933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JlZm9ybWVyLnB5) | `100.00% <ø> (+78.37%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9vcGVuYWkucHk=) | `22.58% <0.00%> (-72.26%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/6933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9hbGJlcnQucHk=) | `21.47% <0.00%> (-69.44%)` | :arrow_down: |
| [src/transformers/tokenization\_xlm\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/6933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25feGxtX3JvYmVydGEucHk=) | `48.80% <0.00%> (-46.43%)` | :arrow_down: |
| [src/transformers/tokenization\_mbart.py](https://codecov.io/gh/huggingface/transformers/pull/6933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWJhcnQucHk=) | `57.14% <0.00%> (-39.69%)` | :arrow_down: |
| [src/transformers/tokenization\_marian.py](https://codecov.io/gh/huggingface/transformers/pull/6933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbWFyaWFuLnB5) | `67.79% <0.00%> (-31.36%)` | :arrow_down: |
| [src/transformers/modeling\_transfo\_xl\_utilities.py](https://codecov.io/gh/huggingface/transformers/pull/6933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190cmFuc2ZvX3hsX3V0aWxpdGllcy5weQ==) | `52.98% <0.00%> (-13.44%)` | :arrow_down: |
| [src/transformers/modeling\_transfo\_xl.py](https://codecov.io/gh/huggingface/transformers/pull/6933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190cmFuc2ZvX3hsLnB5) | `67.10% <0.00%> (-12.67%)` | :arrow_down: |
| [src/transformers/modeling\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/6933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG0ucHk=) | `88.77% <0.00%> (-2.55%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6933/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `80.96% <0.00%> (-1.30%)` | :arrow_down: |
| ... and [8 more](https://codecov.io/gh/huggingface/transformers/pull/6933/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6933?src=pr&el=footer). Last update [e95d262...ca0f022](https://codecov.io/gh/huggingface/transformers/pull/6933?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Perhaps this is not the place to ask, but why do we use rst in some docs and md in others? I am yet to use rst, so I don't know what the cons/pros are. Perhaps it has to do with sphinx's preferred format for its linking features? <|||||>We need rst in the docstrings because that's the format sphinx uses. Then we need to use rst in the doc files that want to link to some functions/classes to be able to leverage sphinx autolinking features. Markdown is also supported, but you can't automatically link to a class/function in it, so I prefer using rst. 

In the source docs most of the files are in rst apart from some simlinks to some READMEs (that need to be in Markdown to properly display on GitHub), the CONTRIBUTING and one file bout migration (this one could be converted to rst if we really wanted to). For a new file, I'd strongly encourage rst unless there is a reason to use Markdown.<|||||>Excellent. I didn't know any of this. Will be adding .rst for new files in the future (though can't help but notice that markdown seems a way easier/more intuitive to write).