Update mutations.md (#642)

Author: KyleAMathews

docs/guides/mutations.md

@@ -203,7 +203,7 @@ Custom actions provide the cleanest way to capture specific types of mutations a
 The mutation lifecycle follows a consistent pattern across all mutation types:
 
 1. **Optimistic state applied**: The mutation is immediately applied to the local collection as optimistic state
-2. **Handler invoked**: The appropriate handler (`onInsert`, `onUpdate`, or `onDelete`) is called to persist the change
+2. **Handler invoked**: The appropriate handler — either `mutationFn` or a Collection handler (`onInsert`, `onUpdate`, or `onDelete`) — is called to persist the change
 3. **Backend persistence**: Your handler persists the data to your backend
 4. **Sync back**: The handler ensures server writes have synced back to the collection
 5. **Optimistic state dropped**: Once synced, the optimistic state is replaced by the confirmed server state