Add docs for inventory caching (#46402)

* Add docs for inventory caching

* Fix subsection headers

* Fix top header

docs/docsite/rst/plugins/cache.rst

@@ -2,20 +2,22 @@
 
 
 Cache Plugins
--------------
+=============
 
 Cache plugin implement a backend caching mechanism that allows Ansible to store gathered facts or inventory source data
 without the performance hit of retrieving them from source.
 
 The default cache plugin is the :doc:`memory <cache/memory>` plugin, which only caches the data for the current execution of Ansible. Other plugins with persistent storage are available to allow caching the data across runs.
 
+You can use a separate cache plugin for inventory and facts. If an inventory-specific cache plugin is not provided and inventory caching is enabled, the fact cache plugin is used for inventory.
 
 .. _enabling_cache:
 
-Enabling Cache Plugins
-++++++++++++++++++++++
+Enabling Fact Cache Plugins
+---------------------------
+
+Only one fact cache plugin can be active at a time.
 
-Only one cache plugin can be active at a time.
 You can enable a cache plugin in the Ansible configuration, either via environment variable:
 
 .. code-block:: shell
@@ -35,10 +37,60 @@ or the Ansible :ref:`configuration <ansible_configuration_settings>` for more de
 A custom cache plugin is enabled by dropping it into a ``cache_plugins`` directory adjacent to your play, inside a role, or by putting it in one of the directory sources configured in :ref:`ansible.cfg <ansible_configuration_settings>`.
 
 
+Enabling Inventory Cache Plugins
+--------------------------------
+
+Inventory may be cached using a file-based cache plugin (like jsonfile). Check the specific inventory plugin to see if it supports caching.
+If an inventory-specific cache plugin is not specified Ansible will fall back to caching inventory with the fact cache plugin options.
+
+The inventory cache is disabled by default. You may enable it via environment variable:
+
+.. code-block:: shell
+
+    export ANSIBLE_INVENTORY_CACHE=True
+
+or in the ``ansible.cfg`` file:
+
+.. code-block:: ini
+
+    [inventory]
+    cache=True
+
+or if the inventory plugin accepts a YAML configuration source, in the configuration file:
+
+.. code-block:: yaml
+
+    # dev.aws_ec2.yaml
+    plugin: aws_ec2
+    cache: True
+
+Similarly with fact cache plugins, only one inventory cache plugin can be active at a time and may be set via environment variable:
+
+.. code-block:: shell
+
+    export ANSIBLE_INVENTORY_CACHE_PLUGIN=jsonfile
+
+or in the ansible.cfg file:
+
+.. code-block:: ini
+
+    [inventory]
+    cache_plugin=jsonfile
+
+or if the inventory plugin accepts a YAML configuration source, in the configuration file:
+
+.. code-block:: yaml
+
+    # dev.aws_ec2.yaml
+    plugin: aws_ec2
+    cache_plugin: jsonfile
+
+Consult the individual inventory plugin documentation or the Ansible :ref:`configuration <ansible_configuration_settings>` for more details.
+
 .. _using_cache:
 
 Using Cache Plugins
-+++++++++++++++++++
+-------------------
 
 Cache plugins are used automatically once they are enabled.
 
@@ -46,7 +98,7 @@ Cache plugins are used automatically once they are enabled.
 .. _cache_plugin_list:
 
 Plugin List
-+++++++++++
+-----------
 
 You can use ``ansible-doc -t cache -l`` to see the list of available plugins.
 Use ``ansible-doc -t cache <plugin name>`` to see specific documentation and examples.