Document BC break (#9143)

Closes #9141

Author: greg0ire

UPGRADE.md

@@ -1,5 +1,12 @@
 # Upgrade to 2.10
 
+## BC Break: `UnitOfWork` now relies on SPL object IDs, not hashes
+
+When calling the following methods, you are now supposed to use the result of
+`spl_object_id()`, and not `spl_object_hash()`:
+- `UnitOfWork::clearEntityChangeSet()`
+- `UnitOfWork::setOriginalEntityProperty()`
+
 ## BC Break: Removed `TABLE` id generator strategy
 
 The implementation was unfinished for 14 years.