Bug 30352: Add a comment to the substatus translation

Bug 30352: Add a comment to the substatus translation

The tric of combining the XPath union operator (|) with the
value-of instruction works as a sort of ternary operation.
The value-of only picks the string value of the first node
from the union in document order.

So, if there is a translation, it will be the first node. If there
is no translation, the original substatus will be used. Note that
there will be no translation for authorised values like Staff
collection, etc.

Test plan:
None. Just comments.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>