Linux CentOS 6.4

In what context did you discover the case is important? The same applies to wsrep_slave_UK_checks.

To answer your question:
I found that it is important as if I set it to wsrep_slave_fk_checks in lower case, the server will not start.

I did see it in SHOW VARIABLES where it is shown as wsrep_slave_fk_checks:

MariaDB [test]> show variables like '%fk%';

+-----------------------+-------+

| Variable_name         | Value |

+-----------------------+-------+

| wsrep_slave_fk_checks | OFF   |

+-----------------------+-------+

1 row in set (0.00 sec)

But when I set it it has to be called wsrep_slave_FK_checks. So this is also a software issue, as well as a documentation issue.

Docs have been updated