[docs] Document max_snapshot_bytes_per_sec and max_restore_bytes_per_sec as available settings for S3 repositories

[n0othing]

The S3 Repository section of the docs doesn't list max_snapshot_bytes_per_sec or max_restore_bytes_per_sec as valid settings. The current documentation might suggest those settings are exclusive to "type": "fs" repositories.

[ywelsch]

@debadair would you be interested in changing the docs here?

[jrodewig]

@ywelsch I'll pick this one up.