After encountering issues with setting up S3 myself, I deem the default provided on `S3_ENDPOINT` as misleading. Before, I have been using a custom hostname, only, resulting in lots of warnings in the log reading `expected :endpoint to be a HTTP or HTTPS endpoint`.
This rewrite documents all supported environment variables for the S3
object storage system, and in addition documents the way that Mastodon
constructs URLs that it hands to clients (for them to obtain objects
from the storage provider).
The documentation of the variables lives entirely in the object-storage page
now, instead of being mixed between that page and the main config page. A link
to the object-storage page has been added to the config page.
