[Cache] Update etag-headers.mdx

Per our findings in CUSTESC-53713

Author: nenizera

src/content/docs/cache/reference/etag-headers.mdx

@@ -47,7 +47,13 @@ The Cloudflare network will take the following actions, depending on the visitor
 
 </table-wrap>
 
-Enabling **Respect Strong ETags** in Cloudflare automatically disables Rocket Loader, Email Obfuscation, Automatic HTTPS Rewrites, and Mirage.
+:::note
+
+Enabling **Respect Strong ETags** in Cloudflare automatically disables Rocket Loader, Email Obfuscation, Automatic HTTPS Rewrites, and Mirage. 
+
+When using **Respect Strong ETags** together with [R2](/r2/), if a request comes in with `Accept-Encoding` header including zstd (Zstandard), Cloudflare will cache the content as zstd.
+
+:::
 
 ### Behavior with Respect Strong ETags disabled