Expand the g_sprintf docs

As requested in bug 576833.

glib/gprintf.c

@@ -100,6 +100,11 @@ g_fprintf (FILE        *file,
  * An implementation of the standard sprintf() function which supports
  * positional parameters, as specified in the Single Unix Specification.
  *
+ * Note that it is usually better to use g_snprintf(), to avoid the
+ * risk of buffer overflow.
+ *
+ * See also g_strdup_printf().
+ *
  * Returns: the number of bytes printed.
  *
  * Since: 2.2