Nms 6659 docu jdbc query monitor

JIRA: http://issues.opennms.org/browse/NMS-6659
 - migrated from wiki: http://www.opennms.org/wiki/JDBCQueryMonitor

Todo Review:
- [ ] Typo and grammar
- [ ] Formatting and conventions
- [ ] Content

We don't have this on all others and add possible errors for copy and paste. So please remove to keep it consistent.