Add docs for datamodule hparams

[s-rog]

📚 Documentation

Add missing docs for #3792

(Possibly depends on #8948 if there are changes)

[s-rog]

I was thinking more about the datamodule page which currently doesn't mention hparams at all.
https://pytorch-lightning.readthedocs.io/en/stable/extensions/datamodules.html
https://github.com/PyTorchLightning/pytorch-lightning/blob/master/docs/source/extensions/datamodules.rst