xz: Don't mention lzip's sync flush marker on the man page

xz: Don't mention lzip's sync flush marker on the man page

The sync flush marker isn't valid in .lz files. The sync flush marker
may be used by lzlib, but the resulting streams are only meant to be
decoded by lzlib itself. lzlib's docs make this clear.

Reported-by: Antonio Diaz Diaz
Link: https://www.mail-archive.com/xz-devel@tukaani.org/msg00700.html
Link: https://www.mail-archive.com/xz-devel@tukaani.org/msg00701.html