The documentation for the Request Parameter APIhttps://lucene.apache.org/solr/guide/7_4/request-parameters-api.html#setting-request-parameters names operations as set/update/unset.
So does the introspection.
However, the code for the SolrConfigHandler#handleParams uses set/update/delete. So, the unset is not a correct description. The manual testing also shows only delete command working.
Furthermore, the format of the delete command is different from others (array, not map), so this should be documented as well.