Documentation: lzo: document part of the encoding

commit d98a0526 upstream.

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>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>