A query like {_id: {$gt: 2}} doesn't mean to find all documents with an _id field that compares greater than the number 2. Instead, it means to field documents where the _id field is a number that compares greater than 2. This implicit restriction of comparison predicates to a particular canonical type is called "type bracketing". If we don't have explicit documentation for this behavior already, then we should add some.

See SERVER-22308 for an example of how this could cause confusion.