Add Docs for default_pool slots (#15997)

Until now this was undocumented, however it was used in:
https://github.com/apache/airflow/blob/aa4713e43f92d3e4c68c3ad00e2d44caaf29aafe/airflow/utils/db.py#L75

This was done in 1.10.4 in 2c99ec6
commit. This commit also updates the name from `non_pooled_task_slot_count` to `default_pool_task_slot_count` as
this is what it actually does.

UPDATING.md

@@ -72,6 +72,16 @@ https://developers.google.com/style/inclusive-documentation
 
 -->
 
+### Default Task Pools Slots can be set using ``[core] default_pool_task_slot_count``
+
+By default tasks are running in `default_pool`. `default_pool` is initialized with `128` slots and user can change the
+number of slots through UI/CLI/API for an existing deployment.
+
+For new deployments, you can use `default_pool_task_slot_count` setting in `[core]` section. This setting would
+not have any effect in an existing deployment where the ``default_pool`` already exists.
+
+Previously this was controlled by `non_pooled_task_slot_count` in `[core]` section, which was not documented.
+
 ## Airflow 2.1.0
 
 ### New "deprecated_api" extra

airflow/config_templates/config.yml

@@ -435,6 +435,15 @@
       type: string
       example: ~
       default: ""
+    - name: default_pool_task_slot_count
+      description: |
+        Task Slot counts for ``default_pool``. This setting would not have any effect in an existing
+        deployment where the ``default_pool`` is already created. For existing deployments, users can
+        change the number of slots using Webserver, API or the CLI
+      version_added: 2.2.0
+      type: string
+      example: ~
+      default: "128"
 
 - name: logging
   description: ~

airflow/config_templates/default_airflow.cfg

@@ -250,6 +250,11 @@ hide_sensitive_var_conn_fields = True
 # extra JSON.
 sensitive_var_conn_names =
 
+# Task Slot counts for ``default_pool``. This setting would not have any effect in an existing
+# deployment where the ``default_pool`` is already created. For existing deployments, users can
+# change the number of slots using Webserver, API or the CLI
+default_pool_task_slot_count = 128
+
 [logging]
 # The folder where airflow should store its log files
 # This path must be absolute

airflow/configuration.py

@@ -162,6 +162,7 @@ class AirflowConfigParser(ConfigParser):  # pylint: disable=too-many-ancestors
         ('operators', 'default_queue'): ('celery', 'default_queue', '2.1.0'),
         ('core', 'hide_sensitive_var_conn_fields'): ('admin', 'hide_sensitive_variable_fields', '2.1.0'),
         ('core', 'sensitive_var_conn_names'): ('admin', 'sensitive_variable_fields', '2.1.0'),
+        ('core', 'default_pool_task_slot_count'): ('core', 'non_pooled_task_slot_count', '1.10.4'),
     }
 
     # A mapping of old default values that we want to change and warn the user

airflow/utils/db.py

@@ -72,7 +72,7 @@ def add_default_pool_if_not_exists(session=None):
     if not Pool.get_pool(Pool.DEFAULT_POOL_NAME, session=session):
         default_pool = Pool(
             pool=Pool.DEFAULT_POOL_NAME,
-            slots=conf.getint(section='core', key='non_pooled_task_slot_count', fallback=128),
+            slots=conf.getint(section='core', key='default_pool_task_slot_count'),
             description="Default pool",
         )
         session.add(default_pool)