The page overviewing geospatial indexing and querying (http://docs.mongodb.org/manual/applications/geospatial-indexes/) has an outdated list of supported GeoJSON objects. It claims:
"MongoDB supports the following GeoJSON objects: Point, LineString, Polygon"
However, the geospatial page on 2dsphere indexing (http://docs.mongodb.org/manual/core/2dsphere/) claims:
"MongoDB supports the following GeoJSON objects: Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon, GeometryCollection".