Improve documentation for downloading specific version of a dataset from cli

I managed to find out that the download command expects the dataset in format `/` or `//`, alowing in this way to specify the version. I didn't find anything about this in the documentation

Start at the documentation for the CLI's download command and check how dataset arguments are currently described. Document both / and // forms, making clear that the latter downloads a specific version.

Written by the indexing model from the issue text.