IO: update documentation for HierarchyIterator::weak_export

The documentation for `HierarchyIterator::weak_export` mentions a feature
that was removed at some point. Another example is used to illustrate its
functionality.

No functional changes.

source/blender/io/common/IO_abstract_hierarchy_iterator.h

@@ -59,9 +59,8 @@ struct HierarchyContext {
    *
    * The export hierarchy is kept as close to the hierarchy in Blender as possible. As such, an
    * object that serves as a parent for another object, but which should NOT be exported itself, is
-   * exported only as transform (i.e. as empty). This happens with objects that are part of a
-   * holdout collection (which prevents them from being exported) but also parent of an exported
-   * object. */
+   * exported only as transform (i.e. as empty). This happens with objects that are invisible when
+   * exporting with "Visible Only" enabled, for example. */
   bool weak_export;
 
   /* When true, this object should check its parents for animation data when determining whether