Add description for custom_testplan input field in workflow

.github/workflows/dispatch_module.yml

@@ -21,6 +21,8 @@ on:
           - 'PHP8.1/MySQL8.0'
         default: 'PHP8.1/MySQL5.7'
         description: 'Limit to one PHP/MySQL combination'
+      # custom_testplan is useful for workflow testing cases in feature branches
+      # for main dev branches like b-X.0.x we should avoid using it
       custom_testplan:
         type: string
         required: true