pgsql: doc: Improve output_plugin_libraries documentation

doc: Improve output_plugin_libraries documentation

Describe the effects of removing the builtin plugins from
output_plugin_libraries. It may not be obvious to DBAs why they might
want to keep those in the allowlist, since it wasn't previously possible
to pick and choose plugins without modifying the Postgres installation
itself.

Reported-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAOYmi%2Bmt5bcV6B0DZR-t1tfpX8wjk8Aj%2BftEd%3DxXuH5qege2%2BQ%40mail.gmail.com
Backpatch-through: 14

Branch
------
REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4c6ff7cb407c0f1586ca6085defd933a706a102d

Modified Files
--------------
doc/src/sgml/config.sgml | 41 ++++++++++++++++++++++++++++++++++++
doc/src/sgml/ref/pg_recvlogical.sgml | 2 +-
2 files changed, 42 insertions(+), 1 deletion(-)