[doc] consistent True/False/None default format
This PR sets True/False/None default format in a consistent manner of backticks.

```
find . -type d -name ".git" -prune -o -type f -exec perl -pi -e 's#\*(True|False|None)\*#`$1`#g' {} \;
```

Fixes: https://github.com/huggingface/transformers/issues/14949

@sgugger 

Done