export-controls: Clarify that the metadata is part of the source

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

content/concepts/export-controls.md

@@ -723,7 +723,8 @@ submitted to the department responsible for export control handling.
 
 ## Package metadata
 
-The metadata is going to be shipped with each binary package and made available
+The metadata is going to be maintained alongside the other packaging metadata
+and sources, to be shipped with each binary package and made available
 at image build time.
 
 The exact format of the metadata is to be defined, but it is going to be based