NMS-6698-Docu-DhcpMonitor

I took the descriptions from the exisiting wiki: http://www.opennms.org/wiki/DHCP_Configuration
I would move the hint in the main section in top of the document as

    CAUTION: If in `extendedMode`, the time required to complete the poll for an unresponsive node is increased by a factor of 3.
    Thus it is a good idea to limit the number of retries to a small number