docs: note node device fields that are read-only

docs: note node device fields that are read-only

As noted by Erik Skultety, we use the same XML schema to report
existing devices and to define new devices. However, some schema
elements are "read-only". In other words, they are used to report
information from the node device driver and cannot be used to define a
new device. Note these in the documentation.

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