Document that `airflow db downgrade` from 3.2 to 3.1.x does not revert SDK-serialized metadata row content

### Under which category would you file this issue? Airflow Core ### Apache Airflow version 3.2.1 ### What happened and how to reproduce it? We upgraded production from **Airflow 3.1.8** to **3.2.1**, hit scheduling issues, then attempted to roll back by: 1. Deploying Airflow **3.1.8** image 2. Running `airflow db downgrade -n 3.1.8` Alembic migrations completed successfully. However, **scheduler and triggerer immediately crashed** on startup with: ``` KeyError: <Encoding.VAR: '__var'> ``` in `BaseSerialization.deserialize`, while loading rows from the metadata database (e.g. during `_schedule_all_dag_runs` or trigger deserialization). ### Environment - **Upgrade path:** 3.1.8 → 3.2.1 → attempted rollback to 3.1.8 - **Database:** PostgreSQL - **Executor:** KubernetesExecutor Example traceback (scheduler): ``` File ".../scheduler_job_runner.py", line 1968, in _schedule_all_dag_runs callback_tuples = [(run, self._schedule_dag_run(run, session=session)) for run in dag_runs] ... File ".../airflow/utils/sqlalchemy.py", line 137, in process_result_value return BaseSerialization.deserialize(value) File ".../airflow/serialization/serialized_objects.py", line 897, in deserialize var = encoded_var[Encoding.VAR] KeyError: <Encoding.VAR: '__var'> ``` Similar failures occur in the triggerer when deserializing `trigger.encrypted_kwargs`. ### Root cause (our analysis) There are **two separate layers** in the metadata DB: | Layer | What downgrade migrations handle | What they do NOT handle | |-------|-----------------------------------|-------------------------| | Schema | Tables, columns, alembic revision | — | | Row content | — | Serialized JSON blobs written while 3.2 was running | While Airflow 3.2.x was running, it wrote metadata using **SDK serde** (see [#59711](https://github.com/apache/airflow/pull/59711) and 3.2.1 release notes — serde moved to `airflow.sdk.serde`). Examples: - `trigger.encrypted_kwargs` - `dag_run.conf` and related serialized columns - Deferre…