Fixed #23278 - Add documentation for django.db.backends.schema logger

related to https://code.djangoproject.com/ticket/23278
"Logs the SQL queries that are executed during schema changes to the database by the migrations framework." (and I'd omit the second sentence). Btw, can you check if this includes queries from data migrations such as RunPython and/or RunSQL. If so, we should amend the sentence I suggested.