docs: Improve documentation for dom0= and dom0-iommu=

docs: Improve documentation for dom0= and dom0-iommu=

Update to the latest metadata style, and discuss the options more
completely where appropriate.

Drop the redundant comment beside parse_dom0_param() - it is already out
of sync with the main documentation.  Also drop the individual
documentation for deprecated options which refer to their newer
versions, for the same reason.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>