docs: Clarify dest requirements of g_utf8_strncpy()

(Minor wording tweak by Philip Withnall.)

https://bugzilla.gnome.org/show_bug.cgi?id=520116

glib/gutf8.c

@@ -421,6 +421,9 @@ g_utf8_pointer_to_offset (const gchar *str,
  * must be valid UTF-8 encoded text. (Use g_utf8_validate() on all
  * text before trying to use UTF-8 utility functions with it.)
  * 
+ * Note you must ensure @dest is at least 4 * @n to fit the
+ * largest possible UTF-8 characters
+ *
  * Returns: @dest
  */
 gchar *