DOC: Note in Migration Guide that PointSet::Clone() now copies its data

- Addressed a comment by Matt McCormick at InsightSoftwareConsortium#4923 (comment)

Documentation/docs/migration_guides/itk_6_migration_guide.md

@@ -97,3 +97,13 @@ Accessing outdated ITKv5 migration scripts
 git worktree add .../ITKv5.4 v5.4.0
 ls ../ITKv5/Utilities/ITKv5Preparation
 ```
+
+Class changes
+-------------
+
+The `Clone()` member function of `itk::PointSet` now does a "deep copy" of its
+data, creating a new instance that has a copy of the points, the point data and
+the region information properties of the original PointSet object. With previous
+ITK versions, `PointSet::Clone()` did not copy any data. (It previously just
+created a default-constructed PointSet object, like `PointSet::CreateAnother()`
+does.)