Add xml doc for parameter in indexer declaration

### Clear and concise description of the problem

Current implementation does not write the xml doc for the parameter in indexer declaration, we should add it.

### 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.