Deprecate settings.yml, and document usage

app/update/settings.py

@@ -1,14 +1,29 @@
 """Manages a TinyPilot update settings file.
 
-TinyPilot currently manages some of its settings in a database and some of its
-settings in a YAML settings file. This module is a wrapper around the YAML
-settings file that allows TinyPilot code to modify it cleanly.
+This module is a wrapper around the YAML settings file (settings.yml) that
+allows TinyPilot code to modify it cleanly.
 
 Typical usage example:
 
     settings = update_settings.load()
     settings.ustreamer_desired_fps = 15
     update_settings.save(settings)
+
+Note that we consider the use of settings.yml to be deprecated nowadays. The
+default place to store settings and configuration is the SQlite database. We
+should only add new properties to settings.yml in exceptional cases, such as:
+- Settings that are supposed to be manually overridden by users
+- Settings that are closely related to already existing legacy settings
+
+Historically, settings.yml was used as we relied on Ansible to manage and
+update the device system state. The canonical way for configuration in Ansible
+is files in YAML format, so a YAML file was an obvious choice here. This
+wrapper module resides in the update package, since settings.yml was only
+processed throughout the update process initially. Over time, we also added a
+few other settings, which are independent of the update context.
+As of mid 2023, we have finished migrating away from Ansible, so there is no
+hard requirement for us to continue using the settings.yml file, except for
+backwards compatibility with existing TinyPilot installations.
 """
 
 import yaml