More on `::` briefly in packages.Rmd

[jimgar]

Hey @joelnitta , as requested moving this (now pretty old, sorry!) PR from your fork to the base repo.

Because it is one of the fundamental benefits of the operator, I thought it would be worth putting in a little bit that explains :: can be used to prevent namespace conflicts. It seems reasonable, because I think novice R users could otherwise believe that using conflicted is the only way to work. But this is just a suggestion.

[github-actions[bot]]

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