PC98 support is experimental - add a note to the user documentation

The issue

PC98 emulation is entirely unsupported, experimental and untested. It was inherited from Dosbox-X and was being worked on by Dosbox-X to try to emulate a popular Japanese system.

There is no acknowledgement of this in any of the documentation.

DBXF Version:

4.0+

Summary

A note will need to be added to the documentation explaining that PC98 emulation is entirely unsupported, experimental and untested. While one option is to remove PC98 support entirely, this seems like an extreme solution to a problem which isn't. Removing PC98 from DBXF would go directly against the project's development strategy of "Just make it work."