doc/api-reference: Add note about the future of the registry API

Since the plan is for apps to eventually ensure they are placed in the
right cgroup and rely on the portal to derive the app ID from the cgroup
name, mention this in the api reference.

doc/api-reference.rst

@@ -17,6 +17,11 @@ This might improve the user experience when the host app was launched in a way
 that doesn't follow the standard. See
 :doc:`org.freedesktop.host.portal.Registry <doc-org.freedesktop.host.portal.Registry>`
 
+Disclaimer: The host app calls to the registry will eventually be replacable
+with the app placing itself in a cgroup named according to specific naming
+conventions. It will be documented in
+:doc:`org.freedesktop.host.portal.Registry <doc-org.freedesktop.host.portal.Registry>` when this will be possible.
+
 All apps have access to the portals below:
 
 .. toctree::