add S3 signature v4 workaround to docs

As per https://lists.apache.org/thread.html/dd59f94d76ae809f83dc36958006974d0a13dc0798856d1d64bb7293@%3Cuser.flink.apache.org%3E, we should add a hint to enable signature v4 for older Hadoop versions to work with S3 (for the non-shaded S3 file systems and regions only accepting v4, e.g. eu-central-1)