Add descriptions for S3 env variables

[jgillich]

Found some of them confusing, so here are some descriptions. Does S3_ENDPOINT override S3_HOSTNAME and S3_PROTOCOL? Wasn't sure about that, could potentailly add a note

Also removed some uses of the word extremely because the circumstances weren't all that extreme :smile:

[Kab1r]

It would be nice if S3_READ_TIMEOUT was documented (added in mastodon/mastodon#14952)

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.

[kpfleming]

I've just opened #1393 which addresses these same issues and many more (including answering some of the questions this PR's creator never got answers to). Maybe that one will get reviewed and merged :-)