[AIRFLOW-3502] Update config template to reflect supporting different…

… Celery pool implementation (apache#5477)

The support to different Celery pool implementation has been added
in apache#4308.

But it's not reflected in the default_airflow.cfg yet, while it's
the main portal of config options to most users.

UPDATING.md

@@ -24,6 +24,17 @@ assists users migrating to a new version.
 ## Airflow Master
 
 
+### `pool` config option in Celery section to support different Celery pool implementation
+
+The new `pool` config option allows users to choose different pool
+implementation. Default value is "prefork", while choices include "prefork" (default),
+"eventlet", "gevent" or "solo". This may help users achieve better concurrency performance
+in different scenarios.
+
+For more details about Celery pool implementation, please refer to:
+- https://docs.celeryproject.org/en/latest/userguide/workers.html#concurrency
+- https://docs.celeryproject.org/en/latest/userguide/concurrency/eventlet.html
+
 ### Removal of `non_pooled_task_slot_count` and `non_pooled_backfill_task_slot_count`
 
 `non_pooled_task_slot_count` and `non_pooled_backfill_task_slot_count`

airflow/config_templates/default_airflow.cfg

@@ -417,6 +417,13 @@ ssl_key =
 ssl_cert =
 ssl_cacert =
 
+# Celery Pool implementation.
+# Choices include: prefork (default), eventlet, gevent or solo.
+# See:
+#   https://docs.celeryproject.org/en/latest/userguide/workers.html#concurrency
+#   https://docs.celeryproject.org/en/latest/userguide/concurrency/eventlet.html
+pool = prefork
+
 [celery_broker_transport_options]
 # This section is for specifying options which can be passed to the
 # underlying celery broker transport.  See: