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>
(cherry picked from commit c652ed4b1dcc204d36921d55120d91c758803632)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 105b9fc3138fd606e850d7efcd28f1269b52c53a)
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 38856cd65b54935f84eaa049050ce42d152f9047)
Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>