Documentation update for Icinga inbuilt checks

[yolanjoy]

Documentation update for Icinga inbuilt checks

Please update Icinga documentation for all data being provided by Icinga inbuilt checks such as "icinga", "cluster", "ido", "cluser-zone" and proper details of each parameter.

[oxzi]

It may be a good idea to list the Performance Data for the embedded Check Commands, as these are in the Icinga domain and do not change based on a third party.