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 let me know what you think of this edit...I moved it up the Limitations section with a sentence that says the session length is not configurable (which I guess would be real "limitation" here). I was thinking this might make sense since it looked like the vendor was asking about changing the session details. If the feature is ever added, then this limitation would be removed in favor of more detailed docs on how to configure it. WDYT?

[paigecalvert]

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