docs: Integrate org.freedesktop.host.portal.Registry

Add a new paragraph to the "For App Developers" page mentioning the
host registry.

Include it in the build system as well.

doc/api-reference.rst

@@ -4,8 +4,16 @@ API Reference
 Portal interfaces are available to sandboxed applications with the default
 filtered session bus access of Flatpak.
 
-Unless otherwise specified, they appear under the bus name ``org.freedesktop.portal.Desktop``
-and the object path ``/org/freedesktop/portal/desktop`` on the session bus.
+Desktop portals appear under the bus name ``org.freedesktop.portal.Desktop``
+and the object path ``/org/freedesktop/portal/desktop`` on the session bus,
+unless specified otherwise.
+
+Apps running on the host system have access to a special interface for
+registering the app with XDG Desktop Portal. Registering a host app with
+XDG Desktop Portal improves the user experience when the app uses portals.
+See :doc:`org.freedesktop.host.portal.Registry <doc-org.freedesktop.host.portal.Registry>`
+
+All apps have access to the portals below:
 
 .. toctree::
    :maxdepth: 1

doc/fix-rst-dbus.py

@@ -20,6 +20,8 @@ def adjust_title(lines):
         adjusted_title = title.replace("org.freedesktop.portal.", "")
     elif title.startswith("org.freedesktop.impl.portal"):
         adjusted_title = title.replace("org.freedesktop.impl.portal.", "")
+    elif title.startswith("org.freedesktop.host.portal"):
+        adjusted_title = title.replace("org.freedesktop.host.portal.", "")
     elif title.startswith("org.freedesktop.background.Monitor"):
         adjusted_title = title.replace(
             "org.freedesktop.background.Monitor", "Background Apps Monitor"

doc/meson.build

@@ -40,6 +40,9 @@ if build_documentation
   foreach i: portal_sources
     all_interfaces_xml += i
   endforeach
+  foreach i: portal_host_sources
+    all_interfaces_xml += i
+  endforeach
   foreach i: portal_impl_sources
     all_interfaces_xml += i
   endforeach