streamfmt option in PXC 8.0 mentions tar while Xtrabackup 8.0 does not support it

Since Xtrabackup 8.0 no longer supports other streaming methods than xbstream, when streamfmt option for SST is set to "tar", SST fails with error:

Invalid --stream argument: tar

The SST options documentation page for PXC 8.0 should reflect the only accepted method in:

https://www.percona.com/doc/percona-xtradb-cluster/8.0/manual/xtrabackup_sst.html#streamfmt

to be consistent with:

https://www.percona.com/doc/percona-xtrabackup/8.0/xtrabackup_bin/xbk_option_reference.html#cmdoption-stream