docs: Be explicit about module scope isolation (#637)

* docs: Be explicit about module scope isolation

Follow-up to #636

Author: salim-b

doc/flatpak-manifest.xml

@@ -483,7 +483,7 @@
                 the build options and a list of sources to download and extract before building.
             </para>
             <para>
-                Modules can be nested, in order to turn related modules on and off with a single key.
+                Modules can be nested, in order to turn related modules on and off with a single key. Module scopes are always isolated, so nested modules do not inherit attributes from parent modules.
             </para>
             <para>
                 These are the properties that are accepted: