Correct the retry_db_transaction docstring about provide_session

[1fanwang]

retry_db_transaction's docstring says:

It should not be used with @provide_session.

Three call sites in airflow-core do exactly that, and have for years:

• renderedtifields.py:241
• dagwarning.py:78
• manager.py:701

Why it matters

The guidance is the opposite of what the code requires, so a contributor who follows it either avoids a valid pattern or stacks the decorators the one way that breaks. That combination is also the only way to retry a read whose session is created for it, which is what those three call sites need. The wording has been there since the decorator landed in #14109 (2021).

What the constraint actually is

Ordering, not incompatibility. The retry wrapper resolves session from the call's arguments when it runs (retries.py:91-96) and rolls it back between attempts, so the session must already be bound. @provide_session therefore goes on the outside. The docstring now says that and shows the shape.

Testing Done

Both orderings, against a real SQLAlchemy session, on a clean checkout of main:

@provide_session
@retry_db_transaction
def provide_session_outer(value: int, *, session: Session = NEW_SESSION) -> int:
    return session.scalar(text("SELECT :v"), {"v": value})


@retry_db_transaction
@provide_session
def retry_outer(value: int, *, session: Session = NEW_SESSION) -> int:
    return session.scalar(text("SELECT :v"), {"v": value})

$ python verify_decorator_order.py
@provide_session OUTER, @retry_db_transaction INNER
  -> OK, returned 7
@retry_db_transaction OUTER, @provide_session INNER
  -> TypeError: session is a required argument for retry_outer

The documented order works and returns the queried value. The order the old docstring implied fails at call time with the TypeError raised at retries.py:96.

Docstring-only change, so there is no behavior to regress and no test added.