The mongo server returns an error if we try to use the $documents stage as first stage of an aggregation run on a collection.

Enterprise test> db.test.aggregate([ { $documents: [{ nb: [1, 2, 3] }] }, { $project: { total: { $sum: ["$nb"] } } }] )
MongoServerError: '$documents' can only be run with {aggregate: 1}

I suggest updating the Behavior section of the $documents page to say so. Especially since the error message from the server is not very clear for someone that ignore the meaning of {{

}}