Minor style improvements for the Flax BERT and RoBERTa examples
1. Use `@nn.compact` rather than `@compact` (as to not make it seem
   like compact is a standard Python decorator.
2. Move attribute docstrings from two `__call__` methods to comments
   on the attributes themselves. (This was probably a remnant from
   the pre-Linen version where the attributes were arguments to
   `call`.)

# What does this PR do?

    Minor style improvements:
    
    1. Use `@nn.compact` rather than `@compact` (as to not make it seem
       like `compact` is a standard Python decorator.
    2. Move attribute docstrings from two `__call__` methods to comments
       on the attributes themselves. (This was probably a remnant from
       the pre-Linen version where the attributes were arguments to
       `call`.)


## Before submitting
- [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/)? No. It's just adjusting the Flax example to the current best practices (I work on Flax)
- [x] Did you make sure to update the documentation with your changes? No doc 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).

It's not clear what the right pattern is for docstrings of dataclass attributes. I went with something pragmatic here. I couldn't find any online references for the "correct Pythonic pattern" here -- LMK if there's another form you prefer.

- [x] Did you write any new necessary tests? No new tests. Exists tests pass.

## Who can review?
cc @LysandreJik @mfuntowicz <|||||>Offline approval from @mfuntowicz!