Update rpc-endpoint.mdx (#635)

bundle can only have one backrun tx

Author: yizhang0901

docs/flashbots-auction/advanced/rpc-endpoint.mdx

@@ -116,8 +116,7 @@ example response:
 
 ### mev_sendBundle
 
-`mev_sendBundle` uses a new bundle format to send bundles to MEV-Share. See the [Understanding Bundles](/flashbots-mev-share/searchers/understanding-bundles) page for more information, or check out the [Sending Bundles](/flashbots-mev-share/searchers/sending-bundles) page for a short guide.
-
+`mev_sendBundle` uses a new bundle format to send bundles to MEV-Share. See the [Understanding Bundles](/flashbots-mev-share/searchers/understanding-bundles) page for more information, or check out the [Sending Bundles](/flashbots-mev-share/searchers/sending-bundles) page for a short guide. Note that as of October 20, 2025, a bundle can now only contain one backrun transaction.
 <SendBundleSpec />
 
 example request: