Add note for int8 to vtkSignedCharArray conversion

Documentation/release/9.3.md

@@ -110,6 +110,9 @@ Changes made since VTK 9.2.0 include the following.
 - OSMesa VTK wheels are now provided. These are available on VTK's official
   channels (the VTK repository's Python index and `vtk.org`), but not PyPI
   because OSMesa conflicts with other OpenGL packages.
+- The numpy adapter (`util.numpy_support`) converts `numpy.int8` arrays to
+  `vtkSignedCharArray` rather than `vtkCharArray`, to ensure that signedness
+  is preserved by the conversion.
 
 [](#changes-rendering)
 ### Rendering