docs: newapi.xsl: Generate overall docs for enums too

docs: newapi.xsl: Generate overall docs for enums too

There are/can be overall docs for enums (e.g.
virDomainModificationImpact) not just individual values. But
these never make it into the generated HTML which is a bit
unfortunate as they can contain valuable information for users.

Generate a block with overall enum documentation, just like we do
for functions.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>