Document the automatic unescaping behavior of annotations dashboard_path

doc/api/metrics_dashboard_annotations.md

@@ -11,6 +11,9 @@ POST /environments/:id/metrics_dashboard/annotations/
 POST /clusters/:id/metrics_dashboard/annotations/
 ```
 
+NOTE: **Note:**
+The value of `dashboard_path` will be treated as a CGI-escaped path, and automatically unescaped.
+
 Parameters:
 
 | Attribute      | Type           | Required | Description                                                                  |