Sup 2161 - Update Artifactory docs wording for Uploading/Downloading via Buildkite-Agent

[AthreyaRay]

We had a customer have an issue when trying to download artifacts across builds when using their Artifactory storage. They thought as they configured their environment variables to use their Artifactory that it would be able to download directly from Artifactory, without specifying the Build ID where the artifact was uploaded.

Updating the document elaborating the same as above.

[buildkite-docs-bot]

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