Highlight that restoring data into alternator MV is not supported

[Michal-Leszczynski]

We already document that schema restoration of a cluster containing alternator tables is not supported - the main reason being scylladb/scylladb#19112, but mixing alternator and CQL frontends might cause some other problems as well.
The same reason also applies to data restoration of alternator MVs - that's because SM does so by re-creating them.
So if their names contain some "illegal" CQL characters, then the restore will fail for sure.

Perhaps it would be possible to restore them without drop and re-create, but by running repair, but that's unsure.
For now we should at least document it.