Actually I was wrong it is in the docs, kind of. According to the index definition page (https://docs.atlas.mongodb.com/atlas-search/index-definitions/#array), you can’t even use autocomplete for an array of strings, let alone an array of docs:
“You can’t use the autocomplete type to index fields whose value is an array of strings.”
FWIW, I’ve got an array of ‘tags’ for my use case. Attempting to refactor it as a string that can be searched using autocomplete instead.