Documentation missing is_decoder
### System Info

Not relevant. Issue is about online documentation.

### Who can help?

@sgugger @stevhliu 

### Information

- [ ] The official example scripts
- [ ] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

The bert documentation [page](https://huggingface.co/docs/transformers/model_doc/bert)  mentions `is_decoder` 5 times but the BertConfig [class documentation](https://huggingface.co/docs/transformers/model_doc/bert) not a single time. 

This probably affects other models as well.

### Expected behavior

BertConfig class documentation should contain an entry for `is_decoder`.
Thanks for flagging. Would you like to open a PR with a fix?