Each filter matches as on getPOICategories, so a code stays itself. Each keeps its best three
codes, merged round-robin and capped at the 10 a search accepts. Codes alone skip the catalog request.
discoverPlaces runs the same lookup for filters.poiCategoryQuery: call this first to decide what to
do about the words that match nothing. To browse every close match instead, use getPOICategories.
Resolves category words, or codes, to the POI categories of one search, and reports the words that match none.