docs: newapi.xsl: Generate docs on generic typedefs

docs: newapi.xsl: Generate docs on generic typedefs

We have plenty of generic typedefs (that basically just alias a
struct, or our popular virXXXPtr). Because we do not generate
HTML docs for it, the documentation is placed at random places,
e.g.: comment from virDomainPtr typedef ("a virDomainPtr is
pointer to a virDomain private structure ...") ends up after
virDomainProcessSignal enum block.

There are some less weird occurrences of this problem (e.g.
virBlkioParameterPtr), but yet - the typedef appears in TOC.

Therefore, generate a block for each typedef and put its
description there.

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