cldice loss documentation

[wyli]

follow up of @jocpae in #5938 (comment), the relevant docstring should be reviewed and added to https://github.com/Project-MONAI/MONAI/blob/dev/docs/source/losses.rst?plain=1

please follow the instructions for changing the docs: https://github.com/Project-MONAI/MONAI/blob/dev/CONTRIBUTING.md#building-the-documentation