Add message to documentation that longformer doesn't support token_type_ids
# What does this PR do?

Fixes #9111. This pull request adds a notice to the Longformer model documentation that it does not have `token_type_ids`, similarly to the message on the [RoBERTa documentation](https://huggingface.co/transformers/model_doc/roberta.html).


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.

@sgugger (documentation)
@patrickvonplaten (longformer)

Thanks for your PR! It looks like you did not run the `make style` command to format properly your changes.<|||||>@sgugger Sorry about that. It should be good now.<|||||>Thanks a lot!