DOCBOOK-APPS: controlling variablelist formatting with pi

[David Cramer <dcramer@broadjump.com>]

I notice that lists.xsl looks for a dbfo pi and uses it instead of the
variablelist.as.blocks to format a variablelist, but I haven't figured
out exactly what the pi should look like. I tried <?dbfo
list-presentation="list"?> but it didn't have any effect. Perhaps this
could be mentioned in the documentation for variablelist.as.blocks.

Thanks,
David