Updated the documentation for pref_free_storage.

core/src/main/res/values-qq/strings.xml

@@ -143,7 +143,7 @@
   <string name="help_11">This is a descriptive message explaining where the Zim files are located after downloading. It is showing on the help screen.</string>
   <string name="pref_storage">{{Identical|Storage}}</string>
   <string name="pref_current_folder">This is showing on the preference settings screen, it shows the currently selected storage in which we are downloading the zim files, whether it is internal or external.</string>
-  <string name="pref_free_storage">This refers to free (unused) storage space, ’’not’’ to free as in free of charge.</string>
+  <string name="pref_free_storage">This refers to free (unused) storage space, ’’not’’ to free as in free of charge. Here %s will be replaced by the free space e.g. 20GB.</string>
   <string name="delete_zim_failed">This message appears in the “Android Toast” as an error message. When there are some files that could not be deleted, due to some reason.</string>
   <string name="tts_pause">{{identical|pause}}</string>
   <string name="tts_resume">{{identical|resume}}</string>