Add the new suggestion according to the meeting with Snowflake (#3)

src/main/thrift/parquet.thrift

@@ -270,6 +270,9 @@ struct Covering {
 /**
  * Bounding box of geometries in the representation of min/max value pair of
  * coordinates from each axis. Values of Z and M are omitted for 2D geometries.
+ * Filter pushdown on geometries are only safe for planar spatial predicate
+ * but it is recommended that the writer always generates bounding box statistics,
+ * regardless of whether the geometries are planar or spherical.
  */
 struct BoundingBox {
   1: required double xmin;