Reflect changes in systemd service for LimitMEMLOCK
Reflect changes in systemd service for LimitMEMLOCK

The LimitMEMLOCK suggestion was removed from systemd service file and
instead users should use an override file, so a comment in the
environment file should be updated to reflect the same.

Relates #27630

vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elasticassandra,vroyer/elasticassandra,vroyer/elassandra,vroyer/elasticassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra