docs: Document full node device xml in formatnode.html.in

docs: Document full node device xml in formatnode.html.in

Some of the node device xml schema was documented in drvnodedev.html.in
rather than in formatnode.html.in. Move all of the schema documentation
to formatnode.html.in and provide reference links from the
drvnodedev.html.in page.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>