pgsql: Document a few caveats in synchronous logical replication.

Document a few caveats in synchronous logical replication.

In a synchronous logical setup, locking [user] catalog tables can cause
deadlock. This is because logical decoding of transactions can lock
catalog tables to access them so exclusively locking those in transactions
can lead to deadlock. To avoid this users must refrain from having
exclusive locks on catalog tables.

Author: Takamichi Osumi
Reviewed-by: Vignesh C, Amit Kapila
Backpatch-through: 9.6
Discussion: https://www.postgresql.org/message-id/20210222222847.tpnb6eg3yiykzpky%40alap3.anarazel.de

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9f7bba2629cf3413638936d0376bb50403144332

Modified Files
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doc/src/sgml/logicaldecoding.sgml | 76 +++++++++++++++++++++++++++++----------
1 file changed, 57 insertions(+), 19 deletions(-)