Covering indexes for array

Our blog in bold mentions this requirement when using covering indexes with arrays:

to create Covering Array Indexes, the array itself MUST also be added to the list of other index keys provided to the CREATE INDEX statement.

Unfortunately our documentation does not mention this requirement. I'm not sure which page is best to put it on:

https://developer.couchbase.com/documentation/server/4.6/indexes/covering-indexes.html
https://developer.couchbase.com/documentation/server/4.6/n1ql/n1ql-language-reference/indexing-arrays.html