Update external_cache docs with other configuration options

There are more places than just the minion config file to define
external job cache configuration settings for minions. This change
updates the docs to include some of those other places and the
order in which they're evaluated if the settings are defined in
more than one place.

Fixes #38762

doc/topics/jobs/external_cache.rst

@@ -89,12 +89,33 @@ A simpler returner, such as Slack or HipChat, requires:
 Step 2: Configure the Returner
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-After you understand the configuration and have the external system ready, add
-the returner configuration settings to the Salt Minion configuration file for
-the External Job Cache, or to the Salt Master configuration file for the Master
-Job Cache.
+After you understand the configuration and have the external system ready, the
+configuration requirements must be declared.
 
-For example, MySQL requires:
+External Job Cache
+""""""""""""""""""
+
+The returner configuration settings can be declared in the Salt Minion
+configuration file, the Minion's pillar data, or the Minion's grains.
+
+If ``external_job_cache`` configuration settings are specified in more than
+one place, the options are retrieved in the following order. The first
+configuration location that is found is the one that will be used.
+
+- Minion configuration file
+- Minion's grains
+- Minion's pillar data
+
+Master Job Cache
+""""""""""""""""
+
+The returner configuration settings for the Master Job Cache should be
+declared in the Salt Master's configuration file.
+
+Configuration File Examples
+"""""""""""""""""""""""""""
+
+MySQL requires:
 
 .. code-block:: yaml