pgsql: doc: Improve explanations when a table rewrite is needed

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: /messages/by-id/00e6eb5f5c793b8ef722252c7a519c9a@oss.nttdata.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/06c4f3ae804e6680ff03a09c1f89dfb1ca3d90e9

Modified Files
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doc/src/sgml/ref/alter_table.sgml | 23 ++++++++++++++++-------
1 file changed, 16 insertions(+), 7 deletions(-)