[doc] remove the implied defaults to :obj:`None`, s/True/ :obj:`True/, etc.
Author: stas00
Pull request: yes

as discussed at https://github.com/huggingface/transformers/pull/6932#issuecomment-687362952
**edit**: I also threw in  :obj:`True/False - anything else?
```
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's|, defaults to :obj:.None.||'  {} \;
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's|, defaults to True|, defaults to :obj:`True`|'  {} \;
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's|, defaults to False|, defaults to :obj:`False`|'  {} \;
```
@sgugger 

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6956?src=pr&el=h1) Report
> Merging [#6956](https://codecov.io/gh/huggingface/transformers/pull/6956?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/eff274d629c95fca459969b530b4ad0da5563918?el=desc) will **decrease** coverage by `6.41%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6956/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6956?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6956      +/-   ##
==========================================
- Coverage   80.02%   73.61%   -6.42%     
==========================================
  Files         161      161              
  Lines       30120    30120              
==========================================
- Hits        24105    22172    -1933     
- Misses       6015     7948    +1933     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6956?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/6956/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JhcnQucHk=) | `94.00% <ø> (ø)` | |
| [src/transformers/configuration\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/6956/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JlcnQucHk=) | `100.00% <ø> (ø)` | |
| [src/transformers/configuration\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/6956/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2VsZWN0cmEucHk=) | `100.00% <ø> (ø)` | |
| [src/transformers/configuration\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/6956/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2ZsYXViZXJ0LnB5) | `100.00% <ø> (ø)` | |
| [src/transformers/configuration\_gpt2.py](https://codecov.io/gh/huggingface/transformers/pull/6956/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2dwdDIucHk=) | `97.29% <ø> (ø)` | |
| [src/transformers/configuration\_lxmert.py](https://codecov.io/gh/huggingface/transformers/pull/6956/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2x4bWVydC5weQ==) | `20.00% <ø> (-80.00%)` | :arrow_down: |
| [src/transformers/configuration\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6956/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX21vYmlsZWJlcnQucHk=) | `97.05% <ø> (ø)` | |
| [src/transformers/configuration\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/6956/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX29wZW5haS5weQ==) | `97.14% <ø> (ø)` | |
| [src/transformers/configuration\_reformer.py](https://codecov.io/gh/huggingface/transformers/pull/6956/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JlZm9ybWVyLnB5) | `21.62% <ø> (-78.38%)` | :arrow_down: |
| [src/transformers/configuration\_retribert.py](https://codecov.io/gh/huggingface/transformers/pull/6956/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX3JldHJpYmVydC5weQ==) | `34.78% <ø> (ø)` | |
| ... and [89 more](https://codecov.io/gh/huggingface/transformers/pull/6956/diff?src=pr&el=tree-more) | |

------

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Great, thanks a lot!