gc: document `--all-commits` option

[efiop]

Per treeverse/dvc#2643

[Suor]

This was never meant to be documented nor used. The goal of --all-commits is a ready to use implementation, we decided to only make it public with a proper UI, which is still undecided.

So this one should be closed.

[shcheklein]

@efiop @Suor 👍 , you should hide it from the command line then.