[Speed] Clarify No HTTP/3 Support to Origin (#17229)

PCX-13843

Author: kyouheicf

src/content/docs/speed/optimization/protocol/http3.mdx

@@ -15,6 +15,12 @@ For more background on HTTP/3, visit the [Learning Center](https://www.cloudflar
 
 :::
 
+:::note
+
+This setting is for connection between the user and Cloudflare. HTTP/3 connection to the origin is not yet supported.
+
+:::
+
 ## Availability
 
 <FeatureTable id="speed.http3" />