Adds note about Global region (cloudflare#23317)

Author: angelampcosta

src/content/docs/health-checks/health-checks-analytics.mdx

@@ -22,6 +22,7 @@ You can evaluate origin uptime, latency, failure reason, and specific event logs
 * **Health Checks By Latency**: Shows average latency – measured in round trip time — for individual origins over time.
 * **Event Log**: Shows individual health check data.
   * Select each record for additional details on **Round trip time**, the **Failure Reason**, the **Average Waterfall** (showing chronological data about request stages), **Response status code**, and more.
+  * Note that **Global** is not a configured region; it represents the aggregated data from all enabled regions.
 
 ## Set up alerts