Update a glossary term

Updates the term's keyword or definition. An absent field keeps its stored value. null clears a nullable field or removes a map entry; other fields reject null. In batches and imports, items match stored items by natural key, stored items absent from the batch are untouched, and nothing else is deleted. A blank CSV cell counts as absent. The term ID and translations are kept when the keyword is renamed. Renaming to a keyword another term already has returns 409. Requires org:context:write.