NMS-6642: Documentation CiscoPingMibMonitor

JIRA: http://issues.opennms.org/browse/NMS-6642
- Migrated from wiki http://www.opennms.org/wiki/CiscoPingMibMonitor

Todo Review:
- [ ] Typo and grammar
- [ ] Formatting and conventions
- [ ] Content
(1, 2c, or 3) 
vs
(`1`, `2c`, or `3`)