Make it explicit that an S3 bucket configured for use with Central must accept abitrary connections

https://forum.getodk.org/t/form-attachment-on-s3-works-in-enketo-but-not-web-forms-or-collect/58352/4?u=ln

Start by reading the linked forum discussion and searching the Sphinx documentation for Central's S3 bucket configuration. Update the relevant guidance so it explicitly states that the bucket must accept arbitrary connections, then verify the documentation builds successfully.

Written by the indexing model from the issue text.