Documentation: lzo: document part of the encoding

Add a complete description of the LZO format as processed by the
decompressor. I have not found a public specification of this format
hence this analysis, which will be used to better understand the code.

Cc: Willem Pinckaers <willem@lekkertech.net>
Cc: "Don A. Bailey" <donb@securitymouse.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>