LP #1700762: [DOC] wsrep-log-debug sst option missing documentation

**Reported in Launchpad by Przemek last update 19-09-2017 10:57:11

A debug option was introduced in PXC 5.7, according to https://www.percona.com/live/17/sites/default/files/slides/Meet%20PXC-5.7.pdf - slide 24.

[sst]
wsrep-log-debug=1

I cannot see this option described in https://www.percona.com/doc/percona-xtradb-cluster/5.7/manual/xtrabackup_sst.html

Please, specify also in exactly which version was it introduced.