Add documentation for BertJapanese
# What does this PR do?

Add documentation for BertJapanese

Regarding #9035


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] 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.
- [ ] 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).
- [ ] Did you write any new necessary tests?


## Who can review?

Model: bert: @LysandreJik
Documentation: @sgugger
Thanks a lot for your help!<|||||>@sgugger @LysandreJik 
Thank you for your quick reviewing this PR!

@sgugger 
Thank you for telling me how to format the code block!

> Thanks a lot for your PR! I made a couple of suggestions. Mostly, the example should follow the syntax
> 
> ```
> >>> code_line_1
> >>> code_line_2
> result
> ```

I've split the big code block into two, and then make all lines prefixed with `>>>`.
Now I think I can understand the format.
However, I wonder why BertTweet and BertGeneration, which I referred to before opening this PR, has code blocks without using `>>>` in them.
Are there any specific reasons? (Could it be because the output is not specifically described?)
Or, may I correct them using `>>>`?
<|||||>We should always have the `>>>` as it allows us to use `doctest` which will test the example (it's been deactivated for a while but we will bring it back to life soon). So if you want to add those to some examples where it's missing, go ahead!

The only instance where we would not want those `>>>` is if we don't want the example to be tested.<|||||>Thanks for the detailed explanation about the prefix!
Now, I would like to add `>>>` to examples without the prefix, as far as I can find (except for which you don't want to be tested).