doc: Note only path-style URL support for S3
doc: Note only path-style URL support for S3

This adds a node to mention that currently only path-style URL's are supported for S3, as per code at:
- https://github.com/restic/restic/blob/aa0faa8c7d7800b6ba7b11164fa2d3683f7f78aa/internal/backend/s3/config.go#L42-L45
- https://github.com/restic/restic/blob/aa0faa8c7d7800b6ba7b11164fa2d3683f7f78aa/internal/backend/s3/config.go#L48-L62
restic/restic,restic/restic