bundle-metadata: Recommend <custom>, not <metadata>

<metadata> was specific to appstream-glib, whereas <custom> is
implemented in the reference implementation libappstream.

Apertis: https://phabricator.apertis.org/T3490Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Justin Kim <justin.kim@collabora.com>
Differential Revision: https://phabricator.apertis.org/D5567

docs/application-bundle-metadata.md

@@ -431,10 +431,13 @@ separate cache files, for several reasons:
 Any metadata keys and values that have not been standardized by the AppStream
 project (for example audio roles that might be used to determine a bundle's
 audio priority) must be represented using [](#extension-points) within the
-AppStream metadata. Until an
-[extension point is standardized][AppStream extension points], we recommend
-using the `<metadata>` element defined and used by the [appstream-glib]
-library.
+AppStream metadata. The formal [AppStream specification][AppStream] does
+not provide an extension point, but the
+[reference implementation][libappstream]
+and [appstream-glib] both provide support for a `<custom>` element
+with `<value>` children. We recommend using that element for extension
+points. See the [Apertis Application Bundle Specification] for
+details.
 
 When a store or built-in app-bundle is added, removed or changed,
 the Apertis platform must update the corresponding cache file.
@@ -569,6 +572,7 @@ requester's AppArmor label (part of the AppArmor context).
 [freedesktop.org Icon Theme Specification]: http://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html
 [GetConnectionCredentials]: https://dbus.freedesktop.org/doc/dbus-specification.html#bus-messages-get-connection-credentials
 [iOS icon sizes]: https://developer.apple.com/library/safari/documentation/UserExperience/Conceptual/MobileHIG/IconMatrix.html
+[libappstream]: https://www.freedesktop.org/software/appstream/docs/api/index.html
 [OEM]: https://wiki.apertis.org/Glossary#oem
 [Namespaces in XML]: https://www.w3.org/TR/REC-xml-names/
 [Permissions]: applications.md#Permissions