Wrong returns of API:get_special_tokens_mask()
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...): Roberta

Language I am using the model on (English, Chinese ...): English

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [x ] my own modified scripts: (give details below)
I use transformers tokenizer, and created mask using API: get_special_tokens_mask.
[My Code](https://github.com/ishikawa-takumi/transformers-sample/blob/master/tokenizer.ipynb)

In [RoBERTa Doc](https://huggingface.co/transformers/model_doc/roberta.html), returns of this API is "A list of integers in the range [0, 1]: 0 for a special token, 1 for a sequence token". But I seem that this API returns "0 for a sequence token, 1 for a special token".
## To reproduce


Steps to reproduce the behavior:

1. Use get_special_tokens_mask().
2.
3.

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
Returns of this API is "A list of integers in the range [0, 1]: 0 for a special token, 1 for a sequence token".

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version:
- Platform:
- Python version:
- PyTorch version (GPU?):
- Tensorflow version (GPU?):
- Using GPU in script?:
- Using distributed or parallel set-up in script?:

Yes, this is https://github.com/huggingface/transformers/issues/1784 come again, the issue is in the doc, not in the code<|||||>Indeed, thanks for raising this issue.