Document that migrations that rename foreign key columns should use immediate foreign key checks

[groue]

... so that SQLite updates the foreign key.

It looks like SQLite automatically migrates foreign keys when tables are renamed, but not when columns are renamed (documentation).

I recently had to rename a foreign key, and I had to run the migration with immediate foreign key checks in order to avoid integrity failures:

// Use immediate foreign key checks so that the foreign key is properly migrated.
migrator.registerMigration("v8", foreignKeyChecks: .immediate) { db in
    try db.rename(table: "foo", to: "bar")
    try db.alter(table: "qux") { t in
        t.rename(column: "fooId", to: "barId")
    }
}

This must be documented, because it is far from obvious.