Revert "Merge branch 'revert-90ed8d47' into 'master'"

What does this MR do and why?

This MR reverts Revert "Merge branch 'ashmckenzie/update-lfs-do... (!159170 - merged), which re-instates Document Pure SSH transfer protocol support (!157904 - merged) now that Support Pure-SSH LFS protocol in gitlab-shell (omnibus-gitlab!7740 - merged) is merged.

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