Documentation: update seq_file

Update descriptions of seq_path() and seq_path_root():
starting with commit v3.2-rc4-1-g02125a8, seq_path_root() no longer
changes the value of root;
starting with commit v3.2-rc7-104-g8c9379e, some arguments of seq_path()
and seq_path_root() are const.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>