Update default.conf

In default.conf tell that
MODULES=( 'loaded_modules' 'additional_module' )
is not supported,
cf. #2727

usr/share/rear/conf/default.conf

@@ -1529,6 +1529,7 @@ LIBS=()
 # (see https://github.com/rear/rear/issues/1355).
 # The setting MODULES=( 'loaded_modules' ) results that
 # only those kernel modules that are currently loaded
+# (so MODULES=( 'loaded_modules' 'additional_module' ) is not supported)
 # get included in the recovery system. This results a noticeable
 # smaller recovery system but on the other hand it means that
 # recovery will only work on same replacement hardware.