document the time the signature is good for
document the time the signature is good for

test plan
- the time should show up at doc/api/file.file_uploads.html

Change-Id: I54eec0e6276d46a960878563a0505ddba7152251
Reviewed-on: https://gerrit.instructure.com/25616
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Bracken Mosbacker <48b846001bca4166dc3fb7711563afc17525921e@instructure.com>
Product-Review: Bracken Mosbacker <48b846001bca4166dc3fb7711563afc17525921e@instructure.com>
QA-Review: Bracken Mosbacker <48b846001bca4166dc3fb7711563afc17525921e@instructure.com>

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