NMS-6698: Documentation DhcpMonitor

- Migrate pull request to OpenNMS devel instead development/docs

JIRA: http://issues.opennms.org/browse/NMS-6698
 - migrated from wiki: http://www.opennms.org/wiki/DHCP_Configuration

Todo Review:
- [x] Typo and grammar
- [x] Formatting and conventions
- [x] Content