pgsql: doc: clarify "logical" replication slots

doc: clarify "logical" replication slots

Also mention that logical replication slots are created by default when
subscriptions are created. This should clarify the text.

Backpatch-through: master

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ada78cd7f80a818de1924180ff5a0647cf59ccfb

Modified Files
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doc/src/sgml/logical-replication.sgml | 15 ++++++++-------
1 file changed, 8 insertions(+), 7 deletions(-)