[python][sphinx] Need to escape '**' in docstring

**Description:**

If there is a '**' in dosctring, it needs to be escaped for Sphinx.

For example:
https://github.com/Azure/azure-rest-api-specs/blob/131b265e5626c867683ddb4770610348fc98d740/specification/batch/Azure.Batch/models.tsp#L1651

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.