Check_MK does have an online documentation for the web config engine WATO.
It has a lot of documentation concerning the rules you can configure.
The hard part is knowing that
- the help feature exists
- is not turned on by default
- where to enable it
First, go to Host & Service Parameters in WATO.
There, you just need to go to the actual rule editor, so lets open the first thing we can find:
Select Check_DNS Service
On this screen, closely examine the upper right corner:
So what about documentation? It's the stack of books!
Click it and it'll change color to indicate inline documentation is now on.
Documentation is shown as a yellow box like here:
The full amount of docs is visible if you enter the rule editor:
If it's distracting to you while editing more complex rules, you can always just turn it off for the moment.
Just remember you need to be in the "right" area of WATO to turn it on and off.
The reason (I guess) is that the docs icon is a property of those areas and registered into multisite for display.