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 <c7254805a17bd5c41e24c3a278fcbf3afcdfb0fa@redhat.com>
Reviewed-by: Erik Skultety <2c14d38fa47c8799f1b9c16280abe27f8edfec6e@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

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