Document /app/manifest.json

This is a nice bit of reproducibility that we shouldn't leave
undocumented.

doc/flatpak-builder.xml

@@ -82,7 +82,10 @@
             </itemizedlist>
 
             After this you will end up with a build of the application in <arg choice="plain">DIRECTORY</arg>, which you can
-            export to a repository with the <command>flatpak build-export</command> command.
+            export to a repository with the <command>flatpak build-export</command> command. If you use the <option>--repo</option>
+            option, flatpack-builder will do the export for you at the end of the build process. When flatpak-builder does the
+            export, it also stores the manifest that was used for the build in /app/manifest.json. The manifest is 'resolved',
+            i.e. git branch names are replaced by the actual commit IDs that were used in the build.
         </para>
         <para>
             At each of the above steps flatpak caches the result, and if you build the same file again, it will start