docs: Mention g_string_ref_new() in g_strdup()

Instead of duplicating strings, using the reference counted version may
be more efficient.

https://bugzilla.gnome.org/show_bug.cgi?id=622721

glib/gstrfuncs.c

@@ -342,6 +342,9 @@ get_C_locale (void)
  * The returned string should be freed with g_free()
  * when no longer needed.
  *
+ * You may want to use g_string_ref_new() in order to use
+ * reference counted strings, instead of duplicating them.
+ *
  * Returns: a newly-allocated copy of @str
  */
 gchar*