ref-manual: add documentation of the variable SPDX_NAMESPACE_PREFIX

The documentation of the variable SPDX_NAMESPACE_PREFIX does not exist.
This variable is used to change the prefix of some links in SPDX docs.

(From yocto-docs rev: 33ed2376b501022daf24003e4e6352a91fcb06ee)

Signed-off-by: BELOUARGA Mohamed <m.belouarga@technologyandstrategy.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/variables.rst

@@ -7925,6 +7925,11 @@ system and gives an overview of their function and contents.
       image), compared to just using the :ref:`ref-classes-create-spdx` class
       with no option.
 
+   :term:`SPDX_NAMESPACE_PREFIX`
+      This option could be used in order to change the prefix of ``spdxDocument``
+      and the prefix of ``documentNamespace``. It is set by default to
+      ``http://spdx.org/spdxdoc``.
+
    :term:`SPDX_PRETTY`
       This option makes the SPDX output more human-readable, using
       identation and newlines, instead of the default output in a