flatpak: Clarify versioning policy

https://phabricator.apertis.org/T8169



Signed-off-by: Ryan Gonzalez <ryan.gonzalez@collabora.com>

content/guides/flatpak.md

@@ -50,7 +50,8 @@ $ flatpak --user install \
 {{% notice warning %}}
 During installation you may be prompted for which version to install. Due to
 incompatibilities between flatpak versions, it is highly recommended you select
-the version corresponding to the Apertis version your system is running.
+the version corresponding to the Apertis version your system is running (see
+[Versioning]({{< ref "#versioning" >}}) for more information).
 {{% /notice %}}
 
 Finally, the demo application can be run by executing the following
@@ -60,6 +61,15 @@ command:
 $ flatpak run org.apertis.hmi.gnome_font_viewer
 ```
 
+## Versioning
+
+The reference runtimes and included demos are built for each stable Apertis
+release, starting with v2022. Use of a runtime version *older* than the host
+version is supported, such as running or developing with runtime v2022 on
+Apertis v2023. However, using runtimes *newer* than your Apertis release (e.g.
+using v2023 on a v2022 system) is **not** supported, because newer runtimes may
+depend on Flatpak functionality that is not available in older Apertis releases.
+
 # Runtime Creation
 
 The reference Apertis runtimes are built using