p.conf: add more comments for the piuparts flags used

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p.conf: restrict tarball updates to the base distributions

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p.conf.piatti: set rescheduling scheme back to normal

Signed-off-by: Andreas Beckmann <anbe@debian.org>