NMS-6698-Docu-DhcpMonitor

JIRA: http://issues.opennms.org/browse/NMS-6698
 - migrated from wiki: http://www.opennms.org/wiki/DHCP_Configuration

Todo Review:
- [ ] Typo and grammar
- [ ] Formatting and conventions
- [ ] Content

Problem: Documentation is incomplete see JIRA issue