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_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5a456034b84e9147e648d512c66f2b2a50bad9f3

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