Updates trigger documentation

- Adds documentation around viewing trigger variables
- Updates trigger variable values image

doc/ci/triggers/README.md

@@ -148,7 +148,7 @@ file. The parameter is of the form:
 variables[key]=value
 ```
 
-This information is also exposed in the UI.
+This information is also exposed in the UI. Please note that _values_ are only viewable by Maintainers.
 
 ![Job variables in UI](img/trigger_variables.png)