Entry points concept: Describe X-Apertis-ServiceExec

Apertis: https://phabricator.apertis.org/T3559Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: André Magalhães <andre.magalhaes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D6336

docs/application-entry-points.md

@@ -220,6 +220,15 @@ The following additional keys are defined in the `[Desktop Entry]` group.
   the [D-Bus Activation][Bundle spec D-Bus activation] section of the
   Apertis Application Bundle Specification.
 
+* `X-Apertis-ServiceExec` (string): A command-line similar to `Exec`
+  that starts the entry point in the background, without
+  implicitly *activating* it (causing it to show a window) if it is
+  a graphical program. For example, entry points that use `GApplication`
+  will usually use the same executable as for `Exec` here, but add the
+  `--gapplication-service` option to it.
+  See the [D-Bus Activation][Bundle spec D-Bus activation] section of the
+  Apertis Application Bundle Specification.
+
 * `X-GNOME-FullName` (localestring): The human-readable full name of the
   application, such as `Rhayader Web Browser`. This key is already used
   by the GLib library, and by desktop environments based on it (such as