Nms 6660 docs jdbc stored procedure monitor

JIRA: http://issues.opennms.org/browse/NMS-6660
 - migrated from wiki: http://www.opennms.org/wiki/JDBC_stored_procedure_monitor

Todo Review:
- [ ] Typo and grammar
- [ ] Formatting and conventions
- [ ] Content



class name or reference to `org.postgresql.Driver`?

Maybe also add a couple of examples for db2, oracle and mysql?