docs: make it clear that Sourcegraph owns the repo folder in "Add repositories already cloned to disk"

Created by: ggilmore

Our "Add repositories already cloned to disk" documentation should emphasize that the process is only intended for sideloading a repository as a on time operation. After that, Sourcegraph owns that location on disk and can do whatever it wants.

See https://github.com/sourcegraph/sourcegraph/issues/2044#issuecomment-467274490