Docs: Variables are pre-filled when running pipeline manually

doc/ci/pipelines/index.md

@@ -132,6 +132,10 @@ Pipelines can be manually executed, with predefined or manually-specified [varia
 You might do this if the results of a pipeline (for example, a code build) are required outside the normal
 operation of the pipeline.
 
+[In GitLab 13.7 and later](https://gitlab.com/gitlab-org/gitlab/-/issues/30101),
+the variables fields are pre-filled with any global variables defined in the
+`.gitlab-ci.yml` file.
+
 To execute a pipeline manually:
 
 1. Navigate to your project's **CI/CD > Pipelines**.