Clarify the connection between xml VTK file extensions and VTKFile tag type

[elrnv]

The XML based VTK files use extensions (e.g. .vtu, .vtp, .vts), which indicate the type of object being represented. This must match the type specified in the VTKFile xml tag (e.g. UnstructuredGrid, PolygonData, StructuredGrid resp.). We should clarify this connection somewhere in the docs, and improve the error message that currently reads:

XML error: The extension of the VTK file doesn't match the type specified in the VTKFile tag

to specify potential resolutions (recommend file extension to match the VTKFile tag or a VTKFile tag to match the extension).

Action items:

• [ ] Include a section in related doc comments (as well as a reference to this section in the export functions) to clarify the necessity for the extension to match the type in the XML file.
• [ ] Update the related error message with more helpful suggestions in how to resolve the issue.