doc: Improve explanations when a table rewrite is needed
Further improvement for commit
11bd8318602. That commit confused
identity and generated columns; fix that. Also, virtual generated
columns have since been added; add more details about that. Also some
small rewordings and reformattings to further improve clarity.
Reviewed-by: Robert Treat <rob@xzilla.net>
Discussion: https://postgr.es/m/
00e6eb5f5c793b8ef722252c7a519c9a@oss.nttdata.com