Aggregate commands like

 {aggregate: "foo", pipeline: [{$documents: [...]}], ...}

 fail with "'$documents' can only be run with {aggregate: 1}". However, a $lookup like

 {$lookup: {from: "foo", pipeline: [{$documents: [...]}], ...}}

 succeeds. While executing this $lookup, we will never look at the foreign collection, so it's confusing to allow it to be specified here. This ticket is to consider erroring for these $lookups, too.