xfsdocs-xml: fix various formatting errors

Fix up a few formatting errors pointed out by Turbo Fredriksson, and while
at it also remove various non-ASCII characters that sneaked in during the
initial conversion

Reported-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>