Add MR title regex example for rules

doc/ci/jobs/job_control.md

@@ -263,6 +263,9 @@ Other commonly used variables for `if` clauses:
   branch. Use when you want to have the same configuration in multiple
   projects with different default branches.
 - `if: '$CI_COMMIT_BRANCH =~ /regex-expression/'`: If the commit branch matches a regular expression.
+- `if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH && $CI_COMMIT_TITLE =~ /Merge branch.*/`:
+   If the commit branch is the default branch and the commit message title matches a regular expression.
+   For example, the default commit message for a merge commit starts with `Merge branch`.
 - `if: '$CUSTOM_VARIABLE !~ /regex-expression/'`: If the [custom variable](../variables/index.md#custom-cicd-variables)
   `CUSTOM_VARIABLE` does **not** match a regular expression.
 - `if: '$CUSTOM_VARIABLE == "value1"'`: If the custom variable `CUSTOM_VARIABLE` is