Add a link to the variable priority override section.

This will be helpful for users looking to see how variables defined in the .gitlab-ci.yml can be overridden via triggers.

changelogs/unreleased/DFredell-master-patch-45053.yml

+---
+title: Add a link to the variable priority override section from triggers page
+merge_request: 25264
+author: DFredell
+type: other

doc/ci/triggers/README.md

@@ -249,6 +249,9 @@ curl --request POST \
   https://gitlab.example.com/api/v4/projects/9/trigger/pipeline
 ```
 
+Trigger variables have the [highest priority](../variables/README.md#priority-of-environment-variables)
+of all types of variables.
+
 ## Using cron to trigger nightly pipelines
 
 >**Note:**