Fixed #25371 -- Added reverse_sql and reverse_code examples to docs.

If you want to include literal percent signs in the query, you have to double

them if you are passing parameters.

The ``reverse_sql`` queries are executed when the migration is unapplied, so

you can reverse the changes done in the forwards queries::

    migrations.RunSQL(

        ["INSERT INTO musician (name) VALUES (%s);", ['Reinhardt']],

        ["DELETE FROM musician where name=%s;", ['Reinhardt']],

    )

The ``state_operations`` argument is so you can supply operations that are

equivalent to the SQL in terms of project state; for example, if you are

manually creating a column, you should pass in a list containing an ``AddField``

instance of :class:`SchemaEditor

<django.db.backends.base.schema.BaseDatabaseSchemaEditor>`.

The ``reverse_code`` argument is called when unapplying migrations. This

callable should undo what is done in the ``code`` callable so that the

migration is reversible.

The optional ``hints`` argument will be passed as ``**hints`` to the

:meth:`allow_migrate` method of database routers to assist them in making a

routing decision. See :ref:`topics-db-multi-db-hints` for more details on

            Country(name="France", code="fr"),

        ])

    def reverse_func(apps, schema_editor):

        # forwards_func() creates two Country instances,

        # so reverse_func() should delete them.

        Country = apps.get_model("myapp", "Country")

        db_alias = schema_editor.connection.alias

        Country.objects.using(db_alias).filter(name="USA", code="us").delete()

        Country.objects.using(db_alias).filter(name="France", code="fr").delete()

    class Migration(migrations.Migration):

        dependencies = []

        operations = [

            migrations.RunPython(

                forwards_func,

            ),

            migrations.RunPython(forwards_func, reverse_func),

        ]

This is generally the operation you would use to create