Schema: allow category to have catDesc and nested category elements

[wlpotter]

The schema is flagging the category elements that have both a catDesc (with human-readable label) and nested category elements, e.g. Biblical Manuscripts > Old Testament. We want to allow both a catDesc and nested categories.

For context, related to #1954, we added human-readable labels for the broader categories of Wright's genre classifications, so we can use these to assist users in finding all biblical manuscripts, not just one sub-class at a time. But this meant adding the human-readable catDesc to the taxonomy which is now flagging the schema