clarify if the @organism attribute on the Pathway element can have more than one organism

[egonw]

Currently, <Pathway organism=""> attribute can hold one or more species names. The semantics is not fully defined. The XML Schema clearly indicates there can be zero or one attribute, but I did not find if the attribute can only contain one or also multiple species.

For example, the following GPML is valid against the XML Schema:

<Pathway xmlns="http://pathvisio.org/GPML/2021" title="linamarin biosynthesis"
  organism="Manihot esculenta, Linum usitatissimum, Lotus japonicus, Manihot esculenta"
>

See https://github.com/pathway-lod/Cyc_to_wiki/actions/runs/19994403679/job/57339792081#step:3:258 and pathway-lod/Cyc_to_wiki#4