PGDOCS - Logical replication GUCs - added some xrefs

Hi hackers.

There is a docs Logical Replication section "31.10 Configuration
Settings" [1] which describes some logical replication GUCs, and
details on how they interact with each other and how to take that into
account when setting their values.

There is another docs Server Configuration section "20.6 Replication"
[2] which lists the replication-related GUC parameters, and what they
are for.

Currently AFAIK those two pages are unconnected, but I felt it might
be helpful if some of the parameters in the list [2] had xref links to
the additional logical replication configuration information [1]. PSA
a patch to do that.

~~

Meanwhile, I also suspect that the main blurb top of [1] is not
entirely correct... it says "These settings control the behaviour of
the built-in streaming replication feature", although some of the GUCs
mentioned later in this section are clearly for "logical replication".

Thoughts?

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[1] 31.10 Configuration Settings -
https://www.postgresql.org/docs/current/logical-replication-config.html
[2] 20.6 Replication -
https://www.postgresql.org/docs/current/runtime-config-replication.html

Kind Regards,
Peter Smith.
Fujitsu Australia