Auth is now required sometimes when downloading products #649
Description
Jira: https://asfdaac.atlassian.net/browse/TOOL-3976
Note: The above link is accessible only to members of ASF.
In authentication.md, and possibly elsewhere, we say that authentication isn't required to download products. However, for the ARIA GUNW de-duplication, if a user requests a product that already exists in the archive, we return a URL to the archived product, which requires authentication when downloading.
We should update the docs to note this.