docs: Span cells if there's not doc text for enum val

docs: Span cells if there's not doc text for enum val

When generating HTML documentation we put enum values into a
table so that we can display the value's name, numerical value
and description (if it has one). Now the last part is problem. If
the value doesn't have description the table row has just two
cells and if it has one the row counts three cells. This makes
HTML engines render the description into very little space - for
instance see:

  html/libvirt-libvirt-domain.html#virDomainMemoryStatTags

We can avoid this problem if we let the cell that corresponds to
numerical value span over two cells if there's no description.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>