cmd ref: add info about cache.shared group and available setting, also in use-cases

[efiop]

Per treeverse/dvc/pull/2298

Need to update https://dvc.org/doc/use-cases/multiple-data-scientists-on-a-single-machine and https://dvc.org/doc/commands-reference/config to include cache.shared config option.

[ryokugyu]

@efiop it's still work in progress.

btw, what does cache.shared does?

I am also working on a use case article #455, maybe they are relatable.

[efiop]

@ryokugyu yep, that PR is still work in progress. Let's get back to it when it is merged. I've just created this issue to not forget about modifying docs.

[efiop]

from @mazzma12 from treeverse/dvc#497 (comment)

Note that the group between users has to be a primary group and not a secondary. This can be changed by running : usermod -g groupName $USER