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