clarify 3d bbox query over 2d geometry (#174)

Author: philvarner

item-search/README.md

@@ -101,8 +101,14 @@ should be returned.
 
 **datetime** The datetime parameter use the same allowed values as the [OAF datetime](http://docs.opengeospatial.org/is/17-069r3/17-069r3.html#_parameter_datetime) parameter. This allows for either a single [RFC 3339](https://datatracker.ietf.org/doc/html/rfc3339) datetime or an open or closed interval that also uses RFC 3339 datetimes. Additional details about this parameter can be found in the [Implementation Recommendations](../implementation.md#datetime-parameter-handling).
 
-**bbox** Represented using either 2D or 3D geometries. The length of the array must be 2\*n where n is the number of dimensions. The array contains all axes of the southwesterly most extent followed by all axes of the northeasterly most extent specified in Longitude/Latitude or Longitude/Latitude/Elevation based on [WGS 84](http://www.opengis.net/def/crs/OGC/1.3/CRS84). When using 3D geometries, the elevation of the southwesterly most extent is the minimum elevation in meters and the elevation of the northeasterly most extent is the maximum.  
-
+**bbox** Represented using either 2D or 3D geometries. The length of the array must be 2\*n where 
+*n* is the number of dimensions. The array contains all axes of the southwesterly most extent 
+followed by all axes of the northeasterly most extent specified in Longitude/Latitude or 
+Longitude/Latitude/Elevation based on [WGS 84](http://www.opengis.net/def/crs/OGC/1.3/CRS84). 
+When using 3D geometries, the elevation of the southwesterly most extent is the minimum elevation 
+in meters and the elevation of the northeasterly most extent is the maximum. When filtering with 
+a 3D bbox over Items with 2D geometries, it is assumed that the 2D geometries are at 
+elevation 0.
 
 ## Response