Add documentation for download portal session length

[alicenstar]

This comes from a question a vendor posed here: https://github.com/replicated-collab/grammatech-replicated/issues/50#issuecomment-2405787343

I'm open to better ways to present/phrase this. Wasn't sure exactly where to put it.

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[paigecalvert]

@alicenstar going to merge this one. LMK if it shouldn't be and I can revert/edit as needed.