Document removal of nlohmannjson from CommonCore public API

Common/Core/vtkAbstractArray.h

@@ -45,6 +45,13 @@
  * Unless `Modified` is called, various cached entities, like array range,
  * map created for `LookupValue` may become obsolete and yield incorrect results.
  *
+ * @warning
+ * In VTK 9.4, new method `nlohmann::json vtkAbstractArray::SerializeValues()` was
+ * introduced which required exposing symbols from
+ * VTK::nlohmannjson library in public API. This method will be removed in VTK 9.5 as it caused
+ * difficulty for downstream projects that linked to a different nlohmannjson. It cannot be
+ * deprecated because doing so prevents fixing the underlying issue.
+ *
  * @sa
  * vtkDataArray vtkStringArray vtkCellArray
  */

Documentation/release/dev/breaking-change-remove-nlohmann-json-from-public-API.md

+# Remove usage of the nlohmannjson library from public facing API
+
+The `nlohmann::json vtkAbstractArray::SerializeValues()` method is now removed
+as the public exposure of VTK's vendored `nlohmannjson` library caused difficulty for downstream projects that link to a different nlohmannjson.
+
+These methods were added in [vtk/vtk!11163](https://gitlab.kitware.com/vtk/vtk/-/merge_requests/11163) which was released in 9.4. That merge
+request has now been reverted. These methods are removed (instead of deprecation) because deprecation would prevent fixing the problem.