Remove bounding sphere description,

Change the "first frame" bounding box information to function based on bind-pose instead.

explainer.md

@@ -225,13 +225,11 @@ additional capabilities relating to the animation timing and view parameters.
 * `entityTransform`: a read-write `DOMMatrixReadOnly` that expresses the current mapping of the view of
 the model contents to the view displayed in the browser. 
 * `boundingBoxCenter`: a read-only `DOMPoint` that indicates the center of the axis-aligned bounding box (AABB)
-of the model contents. If there is an animation present, the bounding box is computed for the first frame of 
+of the model contents. If there is an animation present, the bounding box is computed based on the bind pose of 
 the animation and remains static for the lifetime of the model. It does not update based on a change of
 the `entityTransform`.
 * `boundingBoxExtent`: a read-only `DOMPoint` that indicates the extent of the bounding box of the model
 contents. 
-* `boundingSphereCenter`: a read-only `DOMPoint` that indicates the center of the bounding sphere of the model contents, as it may differ from the bounding _box_ center. 
-* `boundingSphereRadius`: a read-only `double` that indicates the radius of the bounding sphere of the model contents. 
 * `duration`: a read-only `double` reflecting the un-scaled total duration of the animation, if present.
 If there is no animation on this model, the value is 0.
 * `currentTime`: a read-write `double` reflecting the un-scaled playback time of the model animation, if present.