Improve the g_dir_read_name documentation

This function can return NULL in error cases as well. Document
this, and explain how to discriminate the cases.
https://bugzilla.gnome.org/show_bug.cgi?id=639771

glib/gdir.c

@@ -190,6 +190,10 @@ g_dir_open (const gchar  *path,
  * and may vary by file system or other operating-system dependent
  * factors.
  *
+ * %NULL may also be returned in case of errors. On Unix, you can
+ * check <literal>errno</literal> to find out if %NULL was returned
+ * because of an error.
+ *
  * On Unix, the '.' and '..' entries are omitted, and the returned
  * name is in the on-disk encoding.
  *