Added description to some Jenkins git mirror pipeline params

Change-Id: I7a252720b18a3c8d2c1b72d9037c5f5c88187b58
diff --git a/jenkins/client/job/git-mirrors/downstream/init.yml b/jenkins/client/job/git-mirrors/downstream/init.yml
index 649ccdc..6f5a66d 100644
--- a/jenkins/client/job/git-mirrors/downstream/init.yml
+++ b/jenkins/client/job/git-mirrors/downstream/init.yml
@@ -38,3 +38,4 @@
               BRANCHES:
                 type: string
                 default: "{{branches}}"
+                description: "Comma-separated list of branches to mirror"