DOCS: Generated table columns are skipped by logical replication

Hi hackers,

While reviewing another thread that proposes to include "generated
columns" support for logical replication [1] I was looking for any
existing PostgreSQL documentation on this topic.

But, I found almost nothing about it at all -- I only saw one aside
mention saying that logical replication low-level message information
is not sent for generated columns [2].

~~

IMO there should be some high-level place in the docs where the
behaviour for logical replication w.r.t. generated columns is
described.

There are lots of candidate places which could talk about this topic.
* e.g.1 in "Generated Columns" (section 5.4)
* e.g.2 in LR "Column-Lists" docs (section 29.5)
* e.g.3 in LR "Restrictions" docs (section 29.7)
* e.g.4 in the "CREATE PUBLICATION" reference page

For now, I have provided just a simple patch for the "Generated
Columns" section [3]. Perhaps it is enough.

Thoughts?

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[1] https://www.postgresql.org/message-id/flat/B80D17B2-2C8E-4C7D-87F2-E5B4BE3C069E%40gmail.com
[2] https://www.postgresql.org/docs/devel/protocol-logicalrep-message-formats.html
[3] https://www.postgresql.org/docs/devel/ddl-generated-columns.html

Kind Regards,
Peter Smith.
Fujitsu Australia