docs: clarify public fallback RPC limitations

Author: crivacy-io

docs/apps/quickstart/build-app.mdx

@@ -154,9 +154,9 @@ Base is a fast, low-cost Ethereum L2 built to bring the next billion users oncha
     BASE_SEPOLIA_RPC_URL="https://sepolia.base.org"
     ```
 
-    <Note>
-    If `https://sepolia.base.org` is unreachable, use an alternative public endpoint such as `https://base-sepolia-rpc.publicnode.com`. For production apps, use a dedicated RPC provider.
-    </Note>
+    <Warning>
+    `https://sepolia.base.org` and fallback endpoints such as `https://base-sepolia-rpc.publicnode.com` are public RPC endpoints. Use them for local development, tutorials, and testnet experiments only. Public endpoints can be rate-limited or unavailable under load. For production apps, use a dedicated RPC provider or your own Base node.
+    </Warning>
 
     Load the variable and import your deployer key securely.