[DOC] Make clear that wsrep_max_ws_rows has no effect

[Przemek]

The wsrep_max_ws_rows variable does not work, as explained in:
https://bugs.launchpad.net/codership-mysql/+bug/1373909
https://github.com/codership/mysql-wsrep/issues/257
https://bugs.launchpad.net/codership-mysql/+bug/1087614
and it is unclear if it's implementation will ever be finished.

Yet the documentation is now misleading the users:
https://www.percona.com/doc/percona-xtradb-cluster/5.6/wsrep-system-index.html#wsrep_max_ws_rows
https://www.percona.com/doc/percona-xtradb-cluster/5.5/wsrep-system-index.html#wsrep_max_ws_rows
http://galeracluster.com/documentation-webpages/mysqlwsrepoptions.html#wsrep-max-ws-rows

We should make it very clear that such variable was added as intention but never really made to work.

A quick test:

percona1 mysql> select @@version,@@version_comment;
+----------------+---------------------------------------------------------------------------------------------------+
| @@version | @@version_comment |
+----------------+---------------------------------------------------------------------------------------------------+
| 5.6.30-76.3-56 | Percona XtraDB Cluster (GPL), Release rel76.3, Revision aa929cb, WSREP version 25.16, wsrep_25.16 |
+----------------+---------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

percona1 mysql> show status like 'ws%status';
+----------------------+---------+
| Variable_name | Value |
+----------------------+---------+
| wsrep_cluster_status | Primary |
+----------------------+---------+
1 row in set (0.00 sec)

percona1 mysql> select @@wsrep_max_ws_rows;
+---------------------+
| @@wsrep_max_ws_rows |
+---------------------+
| 131072 |
+---------------------+
1 row in set (0.00 sec)

percona1 mysql> update t2 set a=a+1;
Query OK, 599040 rows affected (44.44 sec)
Rows matched: 599040 Changed: 599040 Warnings: 0

[Alexey Zhebel (alexey-zhebel)]

This was fixed according to https://github.com/codership/mysql-wsrep/issues/257

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PXC-1913