cmd-ref: document gc changes

[skshetry]

PR treeverse/dvc#3363 is under review. After this is merged, the docs for gc will need adjustments.

[jorgeorpinel]

A corresponding draft PR in this repo would be best @skshetry, to hep avoid docs going out of sync. Please see https://github.com/iterative/dvc.org/wiki/2.-dvc.org-pull-requests-for-core-repo-changes. Thanks!