Potential bug: Tokens with punctuation are re-tokenized although I've set `is_split_into_words=True`
## Environment info
- `transformers` version: 4.5.1
- Platform: Linux-4.19.112+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.7.10
- PyTorch version (GPU?): 1.8.1+cu101 (False)
- Tensorflow version (GPU?): 2.4.1 (False)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
tokenizers: Hi @LysandreJik!

## Information
I am working on a token classification task where my input is in the following format: 

```
texts = [['Foo', 'bar', '.'], ['Hello', 'world', '.']]
tags = [['B-ENT, 'I-ENT', 'O'], ['O', 'O, 'O']]
```

- **Model:** `bert-large-cased`
- **Tokenizer:** `BertTokenizerFast`. I align tokens with their tags as in this [tutorial](https://huggingface.co/transformers/custom_datasets.html#token-classification-with-w-nut-emerging-entities).

## Problem
Although I've set `is_split_into_words=True` in the tokenizer, tokens containing punctuation are tokenized.

## To reproduce
I reproduced the issue in this [Google Colab notebook](https://colab.research.google.com/drive/1mNJ-T6kOaC5_a8C3T_qCXBNFP5WYC5oi?usp=sharing).

## Expected behavior
Since I've set `is_split_into_words=True`, I would expect the tokenizer to keep the tokens as they are and split them into subwords with `##`. For example, if a token is `'foo(bar'`, I would expect it to stay that way, instead of being split into `['foo', '(', 'bar']`.

Thanks a lot for reading the issue! 
The `is_split_into_words` should be set to skip pre-tokenization (splitting on whitespace), not tokenization. This flag should be set to `True` if you have split your text into individual words, and you're now looking to have each word split into tokens and converted to IDs.

This seems to be unclear from the documentation, we'll work on improving this.<|||||>Makes sense, thank you for the clarification! I'd be happy to work on this if needed.<|||||>Yes, we would welcome such a contribution! I guess we would need to find all occurrences of that `is_split_into_words` parameter and clarify that pre-tokenization isn't tokenization as one could expect. 

We would gladly welcome a PR!<|||||>Great, I will work on this next week! Thanks again for the help!