Update stat_name_handler documentation (#21298)
Update stat_name_handler documentation (#21298)

Previously stat_name_handler was under the scheduler section of the
configuration but it was moved to the metrics section since 2.0.0.
bolkedebruin/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,nathanielvarona/airflow,apache/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,apache/airflow,bolkedebruin/airflow,nathanielvarona/airflow,apache/airflow,lyft/incubator-airflow,apache/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow