docs: migration note for multi_call_entrypoint demotion

Author: dbanks12

docs/docs-developers/docs/resources/migration_notes.md

@@ -9,6 +9,10 @@ Aztec is in active development. Each version may introduce breaking changes that
 
 ## TBD
 
+### [Aztec.nr] `multi_call_entrypoint` demoted from protocol contract
+
+`multi_call_entrypoint` is no longer a protocol contract. Its address is now derived from its artifact rather than hardcoded at `6`, and it is deployed on first use rather than at genesis.
+
 ### [Aztec.nr] `public_checks` demoted from protocol contract
 
 `public_checks` is no longer a protocol contract. Its address is now derived from its artifact rather than hardcoded at `4`. The aztec-nr constant has moved and been renamed (`CANONICAL_PUBLIC_CHECKS_ADDRESS` is now `STANDARD_PUBLIC_CHECKS_ADDRESS` under `crate::standard_addresses`).