pgsql: Better document logical replication parameters

Better document logical replication parameters

Add some cross-links between chapter "20. Server Parameters" and
"31. Logical Replication" regarding the available configuration
parameters, for easier navigation; and some more explanatory text too.

I (Álvaro) chose to duplicate max_replication_slots in Chapter 20,
because it has completely different meanings at each side of the
replication link.

Author: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: vignesh C <vignesh21@gmail.com>
Reviewed-by: samay sharma <smilingsamay@gmail.com>
Discussion: https://postgr.es/m/CAHut+PsESqpy7w3Y6cX98c255ZuCjvipkhKjy6hZBjOv4E6iJA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a8500750ca0acf6bb95cf9d1ac7f421749b22db7

Modified Files
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doc/src/sgml/config.sgml              | 66 ++++++++++++++++++++-------
doc/src/sgml/logical-replication.sgml | 86 ++++++++++++++++++++++++++---------
2 files changed, 114 insertions(+), 38 deletions(-)