If the field does not exist, it will be set to an array containing the
unique values in the argument. If the field is not an array, the query
will yield an error.
See also:
Builder::addManyToSet()
See also:
http://docs.mongodb.org/manual/reference/operator/addToSet/
See also:
http://docs.mongodb.org/manual/reference/operator/each/
Deprecation:
1.1 Use {@link Expr::addToSet()} with {@link Expr::each()}; Will be removed in 2.0