xdg-portal: improve description of extraPortals (#6760)

- Specified more clearly how the extra portals affect the environment

modules/misc/xdg-portal.nix

@@ -35,9 +35,12 @@ in {
       type = types.listOf types.package;
       default = [ ];
       description = ''
-        List of additional portals that should be passed to the
-        `xdg-desktop-portal.service`, via the `XDG_DESKTOP_PORTAL_DIR`
-        variable.
+        List of additional portals that should be added to the environment.
+
+        The directory where the portal definitions have been merged together
+        (likely `~/.nix-profile/share/xdg-desktop-portal/portals`) will get
+        passed to `xdg-desktop-portal.service` via the
+        `NIX_XDG_DESKTOP_PORTAL_DIR` environment variable.
 
         Portals allow interaction with system, like choosing files or taking
         screenshots. At minimum, a desktop portal implementation should be