NMS-6649-Docu-FtpMonitor

I took some text passages (Configuration and usage) from HttpMonitor and changed them to the best of my knowledge.
And I'm not sure, what the FtpMonitor exactly checks. Mabye you can add a more detailed description.


Every sentence in one line: http://docs.opennms.org/guide-doc/1.13.4-SNAPSHOT/#doc-guidelines-gotchas