API: Document DBus APIs provided by Phosh

APIs.rst

@@ -10,7 +10,34 @@ Phone/Messaging APIs
 --------------------
 * TBD
 
+Shell APIs
+----------
+DBus APIs provided by the Phone Shell (:ref:`phosh`):
+
+* `org.gnome.Mutter.DisplayConfig`_ provides display information and
+  configuration:
+
+  * Retrieve information about conntected displays,
+  * Set gamma curve (used for e.g. nightlight)
+  * Set primary monitor
+  * Backlight brightness
+  * Lacks `changing displays <https://source.puri.sm/Librem5/phosh/issues/57>`_
+
+  Mostly used by gnome-settings-daemon.
+
+* `org.gnome.Mutter.IdleMonitor`_ to detect if the session is idle
+  (for e.g. screen lock or power saving). Used by e.g. gnome-session
+  for idle detection.
+
+* `org.gnome.keyring.SystemPrompter`_ provides System modal
+  authentication dialogs (used by e.g. gnupg's pinentry, SIM card pin
+  entry, etc.)
+
+
 Mobile Widgets
 --------------
 * `libhandy reference manual <https://honk.sigxcpu.org/projects/libhandy/doc/>`_
 
+.. _`org.gnome.Mutter.DisplayConfig`: https://source.puri.sm/Librem5/phosh/blob/master/src/monitor/org.gnome.Mutter.DisplayConfig.xml
+.. _`org.gnome.Mutter.IdleMonitor`: https://source.puri.sm/Librem5/phosh/blob/master/src/dbus/org.gnome.Mutter.IdleMonitor.xml
+.. _`org.gnome.keyring.SystemPrompter`: https://gitlab.gnome.org/GNOME/gcr/blob/master/gcr/org.gnome.keyring.Prompter.xml